Proppants With Carbide And/Or Nitride Phases

ABSTRACT

The present invention relates to proppants which can be used to prop open subterranean formation fractions. Proppant formulations are further disclosed which use one or more proppants of the present invention. Methods to prop open subterranean formation fractions are further disclosed. In addition, other uses for the proppants of the present invention are further disclosed, as well as methods of making the proppants.

This application claims the benefit under 35 U.S.C. §119(e) of priorU.S. Provisional Patent Application No. 60/950,534, filed Jul. 18, 2007,which is incorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to proppant materials and other uses forthe proppants. The present invention further relates to methods to makea proppant.

Previously, some proppants were made by Oxane Materials, Inc., so thatthe proppant had a template, such as a template sphere, with a shellwherein the template sphere or core typically provided a lightweight orlow density material that had a void or voids, such as a cenosphere. Theshell that was present preferably was a strength-bearing shell that leadto the overall crush resistance of the proppant once the shellencapsulated or coated the template sphere. This proppant holds muchpromise for addressing many of the disadvantages with previous proppantmaterials. However, if a proppant could be prepared having the same orsimilar lightweight qualities, as well as strength-bearing qualities, asthe core/shell proppant, but without the need to form a shell on thetemplate sphere, this would have numerous advantages with respect tocosts, the amount of material used, and the like. Through a reaction ofthe proppant, such as on the surface, a separate shell is not needed,since the reaction forms a phase that can replace the need of a shell.

Ceramic proppants are widely used as propping agents to maintainpermeability in oil and gas formations. Conventional proppants offeredfor sale exhibit exceptional crush strength but also extreme density.Typical densities of ceramic proppants exceed 100 pounds per cubic foot.Proppants are materials pumped into oil or gas wells at extreme pressurein a carrier solution (typically brine) during the fracturing process.Once the pumping-induced pressure is removed, proppants “prop” openfractures in the rock formation and thus preclude the fracture fromclosing. As a result, the amount of formation surface area exposed tothe well bore is increased, enhancing recovery rates. Proppants also addmechanical strength to the formation and thus help maintain flow ratesover time. Three grades of proppants are typically employed: sand,resin-coated sand and ceramic proppants. Proppants are principally usedin gas wells, but do find application in oil wells.

Relevant quality parameters include: particle density (low density isdesirable), crush strength and hardness, particle size (value depends onformation type), particle size distribution (tight distributions aredesirable), particle shape (spherical shape is desired), pore sizedistribution (tight distributions are desirable), surface smoothness,corrosion resistance, temperature stability, and hydrophilicity(hydro-neutral to phobic is desired).

Ceramic proppants dominate sand and resin-coated sand on the criticaldimensions of crush strength and hardness. They offer some benefit interms of maximum achievable particle size, corrosion and temperaturecapability. Extensive theoretical modeling and practical case experiencesuggest that conventional ceramic proppants offer compelling benefitsrelative to sand or resin-coated sand for most formations.Ceramic-driven flow rate and recovery improvements of 20% or morerelative to conventional sand solutions are not uncommon.

Ceramic proppants were initially developed for use in deep wells (e.g.,those deeper than 7,500 feet) where sand's crush strength is inadequate.In an attempt to expand their addressable market, ceramic proppantmanufacturers have introduced products focused on wells of intermediatedepth.

Resin-coated sands offer a number of advantages relative to conventionalsands. First, resin coated sands exhibit higher crush strength thanuncoated sand given that resin-coating disperses load stresses over awider area. Second, resin-coated sands are “tacky” and thus exhibitreduced “proppant flow-back” relative to conventional sand proppants(e.g. the proppant stays in the formation better). Third, resin coatingstypically increase sphericity and roundness thereby reducing flowresistance through the proppant pack.

Ceramics are typically employed in wells of intermediate to deep depth.Shallow wells typically employ sand or no proppant. As will be describedin later sections, shallow “water fracs”' represent a potential marketroughly equivalent to the current ceramic market in terms of ceramicmarket size.

The family of non oxide based ceramic materials, specifically thecarbides and nitrides of metallic materials, display exceptionalmechanical, thermal and chemical properties all of which in combinationwould be ideal candidates for a proppant system. Although, they displayvery high intrinsic failure strengths, hardnesses, and fracturetoughnesses, their apparent properties are highly dependent upon themicrostructure of the ceramic material that develops during thesintering stage. Significant research has been conducted in thesintering of the carbide and nitride class of materials, the mostimportant of which is the use of a glass forming liquid phase sinteringaid to assist with the densification of the system. Although, the liquidphase sintering approach assists with the densification, the propertiesof such a system are less than optimal and fail to reach the intrinsicproperties that these materials are capable of, due primarily to theeffects of a relatively weak phase existing at the grain boundaries ofthe ceramic material. In addition, with the liquid phase sinteringapproach, a high level of shrinkage occurs during sintering. The degreeof shrinkage is dependent upon a number of parameters, the most criticalof which is particle size. Typically the degree of shrinkage canapproach 20% or higher.

Another approach to improve the sintering and consequently theproperties of such ceramic systems has been with a reaction mechanismthat forms the appropriate carbide and/or nitride phase directly fromthe metallic phase. In this method, a preform of the appropriate metalis produced, with approximately 25-30% percent residual porosity. Thecomponent is then subjected to thermal treatments under the appropriateatmosphere to induce the formation of the carbide or nitride phase.During the formation of the carbide or nitride phase, a volume increaseoccurs, which serves to close the residual porosity and yield a highlydense ceramic body that is more or less pore free. By carefullycontrolling the initial porosity of the preform, the volume expansionassociated with the formation of the carbide or nitride phase willcompletely fill all internal porosity and the outer volume of thepreform will remain unchanged. This process is termed net shape forming.

SUMMARY OF THE INVENTION

A feature of the present invention is to provide a proppant havingsuitable crush strength and/or buoyancy as shown by specific gravity.

A further invention of the present invention is to provide a proppantthat can overcome one or more of the disadvantages described above.

The present invention relates to a method of making a proppant from aproppant material comprising silica. The method includes reducing in areactor at least a portion of the silica of the proppant material tosilicon metal. Then, the process can further include nitriding at leasta portion of the silicon metal or carbiding at least a portion of thesilicon metal or both. The present invention further relates to productsmade by the processes of the present invention.

Also, the present invention relates to a proppant comprising a siliconnitride phase or a silicon carbide phase or both.

The present invention further relates to a method of making a proppantfrom a proppant material comprising at least one metal oxide. The methodincludes reducing in a reactor at least a portion of the metal oxide ofthe proppant material to its respective metal. Then, the process canfurther include nitriding at least a portion of the metal or carbidingat least a portion of the metal or both. The present invention furtherrelates to products made by the processes of the present invention.

Also, the present invention relates to a proppant comprising at leastone metal nitride phase or a metal carbide phase or both.

In addition, the present invention relates to a variety of uses for theproppant as explained herein.

The present invention further relates to a proppant having a surfacethat comprises a silicon nitride and/or silicon carbide, wherein thesurface has an average grain size of 1 micron or less. Other averagegrain sizes are possible. The surface can have a maximum grain sizeand/or a tight distribution with respect to the grain sizes.

The present invention further relates to a method to prop opensubterranean formation fractions using one or more proppants of thepresent invention, which are preferably contained in proppantformulations.

The present invention further relates to methods of making the variousproppants of the present invention.

The present invention also relates to the use of the proppant for theuses described herein, including, but not limited to proppants forhydrocarbon recovery, concrete formulations, composite reinforcementphase, thermal insulating material, electrical insulating material,abrasive material, catalyst substrate and/or support, chromatographycolumn materials (e.g., column packings), reflux tower materials (e.g.,reflux tower packings, for instance, in distillation columns), and thelike.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are intended to provide a further explanation of the presentinvention, as claimed.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to a method of making a proppant from avariety of materials, such as a proppant material comprising silica orother metal oxide. The method involves reducing in a reactor at least aportion of the silica to silicon metal and then nitriding and/orcarbiding at least a portion of the silicon metal.

In the present invention, the reduction and subsequent carbiding ornitriding of the silica phase may follow one of the following reactions;

-   -   a. In the case of a stoichiometric mass of carbonaceous material        added to the silica bearing proppant, the following reaction may        proceed,

SiO₂+C→SiO+CO→SiC

-   -   b. In the case of excess carbonaceous material added to the        silica bearing proppant at the commencement of the reaction,

SiO₂+C→SiO+CO→SiC+C→Si+C→SiC

-   -   c. In the case of a two stage reaction process, where the        initial reaction takes place with a stoichiometric mass of        carbonaceous material, and then a subsequent addition of        additional carbonaceous material,

SiO₂+C_(excess)→SiO+CO→SiC+C_(excess)→Si

And thence;

Si+C_(excess)→SiC

-   -   d. In the case of reduction of the silica bearing phase to        silicon metal followed by nitridation to form the silicon        nitride phase,

SiO₂+C→SiO+CO→SiC+C_(excess)→Si

And thence;

3Si+4N→Si₃N₄

The above reactions are also applicable where a metal oxide in generalis used and the Si would be replaced by the appropriate metal in themetal oxide.

In more detail, the proppant material can be any material that containsat least some silica in the material. For instance, the proppantmaterial can be a material that contains at least 5% silica, such as atleast 10% silica, at least 15% silica, at least 20% silica, at least 25%silica, at least 30% silica, at least 50% silica, at least 75% silica,at least 85% silica, at least 95% silica, wherein all percents are byweight of the material. For instance, the silica content of the proppantmaterial can be from 5% by weight to 99% by weight. The remainingpercent by weight content of the proppant material can be other metaloxides, metals, and/or other elements, oxides, nitrides, carbides, andthe like. Examples of suitable proppant materials include, but are notlimited to, silica (e.g. sand), poraver, glass spheres, cenospheres,hollow glass spheres, glass beads, and the like. The proppant materialcan be in any shape and preferably is in a shape desirable for proppantuse, such as particulates (e.g., spherical). For instance, the proppantmaterial can be in the shape of a sphere, or other shapes as mentionedbelow, for the template material.

The proppant material can have an outer surface that contains silica andthis outer surface can be nitrided or carbided to form an outer surfacethat is a silicon nitride phase or a silicon carbide phase. When silicais on the outer surface of a proppant material and converted to siliconmetal and then nitrided or carbided, this forms a phase around theexterior regions of the proppant material, wherein this outer surface orouter phase can serve as a stronger phase compared to the interiorregion of the proppant material. In converting the silicon metal to asilicon-nitride or silicon-carbide, this provides the ability to form amaterial, such as a ceramic material with improved mechanical and/orphysical properties, without the need to form a separate ceramic shellor other shell on a template material.

Other examples of proppant materials that can be used include, but arenot limited to:

-   -   a. James Hardie's “Synthetic Cenospheres”    -   b. KGF KeraGlas Freiberg GmbH's assorted “KeraPearls” including        Kerabims, KeraGlas, KeraPlus, KeraLight    -   c. Liapor's Reaver product    -   d. Assorted lightweight aggregates, including flyash derived,        lightweight aggregates including:        -   i. Lecat        -   ii. Liapor        -   iii. Others    -   e. Glass beads made by 3M, Potters Beads, Omega Minerals, and        Saint Gobain    -   f. Hy-Tech Materials' synthetic microspheres    -   g. Kinetico's Macrolite    -   h. Noblite    -   i. Periite, and/or    -   j. Brace microspheres as distributed by Harper International in        the USA.

The proppant material can be a composite material. For instance, thecarbiding/nitriding of composite particles (e.g., silica or aluminasolid/foamed/hollow spheres), for example, sized from 0.1 micron to 50.0microns such as those manufactured by:

a. Apollo SRI

b. NanoRidge

c. Oxane Materials

d. Omega Minerals (NanoCil)

e. 3M

f. Saint Gobain

g. Asahi Glass and/or

h. Maprecos.

The level (depth) of the outer surface that is nitrided or carbided canbe controlled when the entire converted silicon metal is converted to asilicon-nitride or silicon-carbide. In other words, the silica that islocated anywhere in the proppant material is converted to a siliconmetal and then to silicon-nitride or silicon-carbide. As an alternative,the exterior region alone can be nitrided or carbided to form asilicon-nitride or silicon-carbide phase, wherein the interior regioncontains silica and/or silicon metal that is not converted to asilicon-nitride or silicon-carbide.

The depth of converting the silica to silicon metal and then tosilicon-nitride or silicon-carbide from the exterior surface to the mostcentral area of the proppant material can be controlled such that theconversion can be limited to any desirable depth. For instance, for agiven radius of a proppant material, the depth of converting from thesurface inward can be 0.1% to 100% of the radius, such as 0.2% to 90%,0.5% to 80%, 1% to 70%, 2% to 60%, 5% to 50%, 7.5% to 40%, 10% to 30%,15% to 25% of the radius. If the proppant material is not spherical,then these same percentages can apply to the average thickness of theproppant material or average length of the proppant material or thedimensions of the proppant material. The depth of converting can be inthickness from 0.01 microns or more, such as 0.1 micron to 200 microns,1 micron to 150 microns, 5 microns to 125 microns, 15 microns to 100microns, and the like.

For instance, 5% or more, 10% or more, 15% or more, 20% or more, 30% ormore, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more,90% or more, or 100% of the silica present in the proppant material canbe converted to silicon and then either to a silicon-nitride and/orsilicon-carbide, wherein the percent is with reference to percent byweight of the overall silica present in the proppant material.

The reactor can be any vessel which would permit the method of thepresent invention to be achieved. For instance, the reactor can be afluidized bed furnace or fluidized furnace. The reactor can be a hightemperature reactor, for instance, with process atmospheric control(s).Other types of furnaces can be used. The high temperature reactor can bea sealed chamber that permits control of the process atmosphere(composition, pressure, and the like) and can be heated by any means,including but not limited to radiant, infra-red, microwave, induction,RF, laser, self propagating combustion, and the like. The fluidized bedfurnace can use air or oxygen or an inert gas as the initial fluidizingmedium. Other gases can be used as the fluidizing medium. The fluidizedmedium can be a non-oxidizing, oxygen-free fluid, which is optionallypre-heated. Other possible furnaces (or reactors) can include:

-   -   i. Rotary    -   ii. Static Bed (or other dynamic bed furnace)    -   iii. Muffled    -   iv. Drop Tower    -   v. Mechanical fluid bed where the air is recycled and/or    -   vi. Microwave,        -   These above furnaces generally use a sealed environment.    -   vii. Conventional fluidized bed furnace.

The process can involve a reducing step or multiple reducing steps whichcan comprise utilizing at least one reducing agent in the presence ofthe proppant material comprising silica. This reducing agent ispreferably in the form of a gas. The reducing agent (e.g., reducing gas)can be initially present with the proppant material in the reactor orcan be subsequently introduced and can optionally replace any or all ofthe fluidizing medium that is initially present in the reactor. Examplesof suitable reducing agents include, but are not limited to, carbonmonoxide, hydrogen, or other gases capable of serving as oxygen gettersto remove oxygen from the silica. The ratio of carbon monoxide to silicacan be approximately 5-15 parts by weight CO to 1 part silica by weight.Other amounts can be used depending upon the amount of silica present inthe material that needs to be reduced to silicon metal. Similar amountscan be used for other reducing agents.

After at least a portion of the silica in the proppant material isreduced, the material can be subjected to nitriding or carbiding. Forinstance, the nitriding can use any nitrogen-containing material, suchas a nitrogen-containing gas. The nitrogen-containing gas can also serveas the fluidizing medium that replaces the previously present fluidizingmedium, such as the carbon monoxide or air. The nitriding can occur withany nitrogen-containing gas, for instance, pure nitrogen, anhydrousammonia gas, nitrogen oxides, or other gases. Optionally, hydrogen gasand/or helium gas can be additionally used with the nitrogen-containinggas.

The air that can be used as at least an initial fluidizing medium can bepre-heated, for instance, at a temperature of from about 25° C. to about1,000° C.

As an option, the method can further comprise introducing at least onecarbonaceous material before, during, and/or after the reducing agent ispresent. The carbonaceous material can be any carbonaceous materialsufficient to promote a catalytic or faster reaction so that the overallreduction of the silica present in the proppant material occurs at afaster rate. Also, the carbonaceous material can alternatively oradditionally provide an additional source of carbon monoxide to serve asthe fluidizing medium and reducing agent. In the alternative or inaddition, the presence of carbonaceous material can provide a source ofcarbon to form a silicon carbide in the proppant material. Thecarbonaceous material can be pulverized carbonaceous material. Thecarbonaceous material can be petroleum coke, charcoal, graphite, carbonblack, activated carbon, and the like. Further examples include anyother material reduced to elemental carbon or that can be reduced toelemental carbon, such as biomass materials that can include, but arenot limited to, cellulose materials, hardwood sawdusts, softwoodsawdusts, wood flour, starch, starch granules, potato granules,pulverized seeds, pulverized seed hulls, pulverized grasses and treewastes, pulverized coconut shells and/or fibers, pulverized rice hulls,pulverized grains such as wheat, corn, barley, oats, rye, buckwheat, andthe like. In the case of reducing the silica to silicon metal, thecarbon:silica ratio can be greater than 2 and preferably less than 6.From 2-6 parts of carbon to 1 part silica by weight can be used. Otheramounts can alternatively be used. In the case of carbonizing thesilicon metal to silicon carbide, the carbon:silicon ratio can begreater than 1 and preferably less than 2. From 1-2 parts carbon to 1part silicon by weight can be used. Other amounts can alternatively beused.

In the case of nitriding the silicon metal to silicon-nitride, thenitrogen:silicon ratio can be greater than 2 and preferably less than 5.From 2-5 parts nitrogen to 1 part silicon by weight can be used. In theabove ranges, the amounts of carbon or nitrogen used are based onelemental carbon or elemental nitrogen content.

The carbonaceous material can be at least partially combusted during themethod. This partial combustion can lead to the carbon being convertedto carbon monoxide, thus supplementing the carbon monoxide used asfluidizing medium and reacting gas. An additional benefit to thiscombustion of the carbon in the furnace chamber allows a reduction ofthe heat input into the system from the external heating elements of thefurnace, and with careful control of furnace conditions and reactantflows, the system can become self-propagating and self-sustaining. As anoption, a secondary supply of air (or oxygen-containing gas, such asoxygen) may be introduced into the furnace chamber to effect combustionof a portion of the carbon to carbon monoxide. This air or oxygen can bepre-heated prior to being introduced into the reactor. Thus, prior toand/or during the reducing, an oxygen-containing gas can be introducedinto the reactor to partially combust the carbonaceous material (forinstance, that is present in excess of the stoichiometric requirementsof the reaction). As an option, at least a portion of the carbonaceousmaterial can react with at least a portion of the silicon metal to forma silicon-carbide phase.

In the present invention, the reducing step and/or nitriding step and/orcarbiding step can be repeated any number of times using the same ordifferent reducing agents, carbiding agents, and/or nitriding agentsand/or same or different conditions.

For any of the embodiments of the present invention involving metaloxides, the degree or depth or amount of nitriding and/or carbiding canbe controlled, for instance, by controlling the pressure, temperature,and/or time of one or more steps in the reactor for each step, namelythe reducing step, nitriding step, and/or carbiding step, if used. Forinstance, the following are examples of process parameters. When thetemperature, pressure, and/or time is used on the lower side of theseranges, this will lead to partial reduction, partial carbiding, orpartial nitriding (of the overall proppant material), and when theparameters of temperature, pressure, and/or time are on the upper sideof the ranges provided, this will lead to nearly full or completereduction, nearly complete or fully carbiding, and/or nearly complete orfully completed nitriding of the proppant material with respect to themetal oxide(s) present.

1. Examples of Reducing, Carburizing and Nitriding Process Parameters.

-   -   a. Reduction:        -   i. Temperature: 1000° C. to 1800° C.        -   ii. Pressure: 0.5 to 10 PSIG (in reaction chamber)        -   iii. Time: 60 to 720 minutes (time dependant upon reaction            temperature, reactant pressure, and dimensions of material            to be reduced e.g. wall thickness, etc)    -   b. Carburizing:        -   i. Temperature: 1000° C. to 1800° C.        -   ii. Pressure: 0.5 to 10 PSIG (in reaction chamber)        -   iii. Time: 60 to 720 minutes (time dependant upon reaction            temperature, reactant pressure, and dimensions of material            to be carburized e.g. wall thickness, etc)    -   c. Nitriding:        -   i. Temperature: 800° C. to 1800° C.        -   ii. Pressure: 0.5 to 10 PSIG (in reaction chamber)        -   iii. Time: 60 to 720 minutes (time dependant upon reaction            temperature, reactant pressure, and dimensions of material            to be nitrided e.g. wall thickness, etc)

These ranges are merely examples of various ranges that can be used andother temperatures, other pressures, and other times above and belowthese ranges can be used to achieve various degrees of reduction,carbiding, and nitriding. For purposes of the present invention, thereference to temperatures for purposes of the reducing step, carbidingstep, and/or nitriding step is a reference to the temperature inside thefurnace.

In a further embodiment of the present invention, a silica material(e.g., finely divided) may be optionally coated over (on to) a polymericshape former. Upon heat treatment of the silica coated polymeric shapeformers in an inert atmosphere, the polymer undergoes pyrolysis to formcarbon, which is then used to reduce and carburize the silica shell fromthe inside out.

In a further embodiment of the present invention, a silica material(e.g., finely divided) may be coated over a carbonaceous shape former.Upon heat treatment of the coated carbonaceous material, the carbonreacts with the silica to form silicon carbide from the internal regionsof the shell in an outward direction. The carbonaceous shape former mayalso be a biomass material that undergoes pyrolysis during the heattreatment stage. Examples of carbonaceous and biomass materials can bethose mentioned above.

As an option, the present invention can relate to a method of making aproppant from a proppant material comprising silica, wherein the methodcomprising carbiding at least a portion of the silica to form theproppant. The carbiding of the silica generally will lead to asilicon-carbide phase. Generally, in this method, the intermediateforming of a silicon metal can be avoided, and one can directly carbideby using reactor conditions that are typically less than about 1800° C.The various details of carbiding, as previously described above, equallyapply to this embodiment, such as the controlling of the level ofcarbiding, the types of materials used, and all other parameters asmentioned above. Thus, carbiding without reducing first to the metal,but instead going straight to the carbide can be achieved and cantypically occur around 1800° C. or less, such as 1500° C. or less,according to Ellingham Diagrams.

Furthermore, the present invention relates to a controlled process toform proppants, wherein the process permits one to control the level ofnitriding and/or carbiding as described above and further permits one tomake a proppant that has comparable surface strength, such as crushstrength, and yet be lightweight and have various other beneficialproperties, such as the ability to achieve a wide range of specificgravities, such as from 0.01 g/cc to about 3.2 g/cc, as well as otherproperties, such as a crush strength of from 1,000 to 20,000 psi orhigher (e.g., 1,000 to 15,000 psi, 3,000 to 15,000 psi, 5,000 to 10,000psi), wherein crush strength is determined according to API PRACTICE 60(2^(nd) Ed., December 1996).

The present invention further relates to proppant products formed by theabove processes. Further, the present invention relates to a proppantthat comprises a silicon-nitride phase(s), a silicon-carbide phase(s),or both. The proppant can optionally include a separate silica phase(s),a separate silicon metal phase or phases, and/or can contain otherphases. Other components can be present in the proppant, such as metalcomponents, metal oxide components, nitrides, carbides, and the like.Examples of other materials that can additionally form the proppant ofthe present invention are described below with respect to the templatematerial described herein.

The proppant that has the silicon-nitride phase(s) or thesilicon-carbide phase(s) or both can have one or both of these phasesprimarily on the external regions of the proppant material. These phasescan be located primarily on the surface of the proppant material. If theproppant material is a sphere (or other similar shape) having a radius,as one option, the silicon-carbide phase and/or silicon-nitride phasecan be primarily located from the surface to half-way to the center ofthe sphere, from the surface to one-third of the radius or from thesurface to one-fifth of the radius or from the surface to one-tenth ofthe radius or from the surface to two-thirds of the radius or from thesurface to 90% of the radius, and so on. The depth or degree of thenitride or carbide can range from 1% to 100% of the radius, and beuniform or non-uniform around the proppant surface area and/orinternally.

Other materials that form the proppant material can, in addition, beconverted to a nitride and/or carbide. For instance, other metal oxidesthat may form the proppant material can be converted to their respectivemetal and/or can be converted to their respective metal-nitride phase(s)or metal-carbide phase(s) or both. Thus, the various other componentsthat can be present, as described below for the template material, canbe in their current state as mentioned below, or can be converted totheir metal state, and/or can be converted to a nitride or carbide statebased on the methods of the present invention described above.

It is to be understood that the description above involving silicaequally applies to metal oxides in general, as further described below.

Further, the present invention relates to a method of making a proppantfrom a proppant material comprising at least one metal oxide, whereinthe method comprises reducing in a reactor at least a portion of themetal oxide to its respective metal and nitriding and/or carbiding atleast a portion of the metal to form said proppant. The presentinvention further relates to a method of making a proppant from aproppant material comprising at least one metal oxide, wherein themethod comprises carbiding at least a portion of the metal oxide to formsaid proppant. The carbiding will generally convert the metal oxide to ametal carbide. Preferred metal oxides that can be used in this inventioninclude, but are not limited to, any oxide of any metal in the PeriodicTable. Examples include, but are not limited to, aluminum oxides,antimony oxides, boron oxides, cadmium oxides, cobalt oxides, copperoxides, gold oxides, iron oxides, lead oxides, lithium oxides, magnesiumoxides, manganese oxides, molybdenum oxides, nickel oxides, platinumoxides, potassium oxides, silver oxides, tin oxides, titanium oxides,tungsten oxides, zinc oxides, and/or zirconium oxides. The proppant cancontain more than one metal oxide that can benefit from this method ofthe present invention, such as at least two metal oxides, at least threemetal oxides, at least four metal oxides, at least five metal oxides,and the like. Any combination of the above-identified metal oxides canbe present in the proppant and can be reduced, nitrided, and/or carbidedas an option. More detail is provided below.

The present invention relates to a method of making a proppant from avariety of materials, such as a proppant material comprising at leastone metal oxide. The method involves reducing in a reactor at least aportion of the metal oxide to its respective metal metal and thennitriding and/or carbiding at least a portion of the respective metal.

In more detail, the proppant material can be any material that containsat least one metal oxide in the material. For instance, the proppantmaterial can be a material that contains at least 5% metal oxide, suchas at least 10% metal oxide, at least 15% metal oxide, at least 20%metal oxide, at least 25% metal oxide, at least 30% metal oxide, atleast 50% metal oxide, at least 75% metal oxide, at least 85% metaloxide, at least 95% metal oxide, wherein all percents are by weight ofthe material. The % can be for one or more or total content of metaloxides present in the proppant material. For instance, the metal oxidecontent of the proppant material can be from 5% by weight to 99% byweight. The remaining percent by weight content of the proppant materialcan be other metal oxides, metals, other elements, and/or otherelements, oxides, nitrides, carbides, and the like. Examples of suitableproppant materials include, but are not limited to, metal oxides(aluminum oxides, titanium oxides, zinc oxides, zirconium oxides,silica), poraver, glass spheres, cenospheres, hollow glass spheres,glass beads, and the like. The proppant material can be in any shape andpreferably is in a shape desirable for proppant use, such asparticulates. For instance, the proppant material can be in the shape ofa sphere, or other shapes as mentioned below, for the template material.

The proppant material can have an outer surface that contains at leastone metal oxide and this outer surface can be nitrided or carbided toform an outer surface that is a respective metal nitride phase or arespective metal carbide phase. When at least one metal oxide is on theouter surface of a proppant material and converted to its respectivemetal and then nitrided or carbided, this forms a phase around theexterior regions of the proppant material, wherein this outer surface orouter phase can serve as a stronger phase compared to the interiorregion of the proppant material. In converting the respective metal to arespective metal-nitride or respective metal-carbide, this provides theability to form a material, such as a ceramic material with improvedmechanical and/or physical properties, without the need to form aseparate ceramic shell or other shell on a template material.

Other examples of proppant materials that can be used include, but arenot limited to:

-   -   a. James Hardie's “Synthetic Cenospheres”    -   b. KGF KeraGlas Freiberg GmbH's assorted “KeraPearls” including        Kerabims, KeraGlas, KeraPlus, KeraLight    -   c. Liapor's Reaver product    -   d. Assorted lightweight aggregates, including flyash derived,        lightweight aggregates including:        -   i. Lecat        -   ii. Liapor        -   iii. Others    -   e. Glass beads made by 3M, Potters Beads, Omega Minerals, and        Saint Gobain    -   f. Hy-Tech Materials' synthetic microspheres    -   g. Kinetico's Macrolite    -   h. Noblite    -   i. Perlite, and/or    -   j. Brace microspheres as distributed by Harper International in        the USA.

The proppant material can be a composite material. For instance, thecarbiding/nitriding of composite particles (e.g., metal oxide or aluminasolid/foamed/hollow spheres), for example, sized from 0.1 micron to 50.0microns such as those manufactured by:

a. Apollo SRI

b. NanoRidge

c. Oxane Materials

d. Omega Minerals (NanoCil)

e. 3M

f. Saint Gobain

g. Asahi Glass and/or

h. Maprecos.

The level (depth) of the outer surface that is nitrided or carbided canbe controlled when the entire converted respective metal is converted toa respective metal-nitride or respective metal-carbide. In other words,the at least one metal oxide that is located anywhere in the proppantmaterial is converted to a respective metal and then to the respectivemetal-nitride or respective metal-carbide. As an alternative, theexterior region alone can be nitrided or carbided to form a respectivemetal-nitride or respective metal-carbide phase, wherein the interiorregion contains metal oxide and/or its respective metal that is notconverted to a respective metal-nitride or respective metal-carbide.

The depth of converting the metal oxide to respective metal and then torespective metal-nitride or respective metal-carbide from the exteriorsurface to the most central area of the proppant material can becontrolled such that the conversion can be limited to any desirabledepth. For instance, for a given radius of a proppant material, thedepth of converting from the surface inward can be 0.1% to 100% of theradius, such as 0.2% to 90%, 0.5% to 80%, 1% to 70%, 2% to 60%, 5% to50%, 7.5% to 40%, 10% to 30%, 15% to 25% of the radius. If the proppantmaterial is not spherical, then these same percentages can apply to theaverage thickness of the proppant material or average length of theproppant material or the dimensions of the proppant material. The depthof converting can be in thickness from 0.01 microns or more, such as 0.1micron to 200 microns, 1 micron to 150 microns, 5 microns to 125microns, 15 microns to 100 microns, and the like.

For instance, 5% or more, 10% or more, 15% or more, 20% or more, 30% ormore, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more,90% or more, or 100% of the metal oxide present in the proppant materialcan be converted to respective metal and then either to a respectivemetal-nitride and/or respective metal-carbide, wherein the percent iswith reference to percent by weight of the overall metal oxide presentin the proppant material.

The reactor can be any vessel which would permit the method of thepresent invention to be achieved. For instance, the reactor can be afluidized bed furnace or fluidized furnace. The reactor can be a hightemperature reactor, for instance, with process atmospheric control(s).Other types of furnaces can be used. The high temperature reactor can bea sealed chamber that permits control of the process atmosphere(composition, pressure, and the like) and can be heated by any means,including but not limited to radiant, infra-red, microwave, induction,RF, laser, self propagating combustion, and the like. The fluidized bedfurnace can use air or oxygen or an inert gas as the initial fluidizingmedium. Other gases can be used as the fluidizing medium. The fluidizingmedium can be a non-oxidizing, oxygen-free fluid, which is optionallypre-heated. Other possible furnaces (or reactors) can include:

-   -   i. Rotary    -   ii. Static Bed (or other dynamic bed furnace)    -   iii. Muffled    -   iv. Drop Tower    -   v. Mechanical fluid bed where the air is recycled and/or    -   vi. Microwave,        -   These above furnaces generally use a sealed environment.    -   vii. Conventional fluidized bed furnace.

The process can involve a reducing step or multiple reducing steps whichcan comprise utilizing at least one reducing agent in the presence ofthe proppant material comprising at least one metal oxide. This reducingagent is preferably in the form of a gas. The reducing agent (e.g.,reducing gas) can be initially present with the proppant material in thereactor or can be subsequently introduced and can optionally replace anyor all of the fluidizing medium that is initially present in thereactor. Examples of suitable reducing agents include, but are notlimited to, carbon monoxide, hydrogen, or other gases capable of servingas oxygen getters to remove oxygen from the metal oxide. The ratio ofcarbon monoxide to metal oxide can be approximately 5-15 parts by weightCO to 1 part metal oxide by weight. Other amounts can be used dependingupon the amount of metal oxide present in the material that needs to bereduced to respective metal. Similar amounts can be used for otherreducing agents.

In a further embodiment of the present invention, the metal oxide thatis to be reduced and reacted to form the carbide and/or nitride may beapplied to a template or shape former, such as via a spray process. Inthe spray process a finely divided metallic oxide, ideally sub micron inmean size, is incorporated into an aqueous slurry and then spray coatedonto the template using any one of a number of methods, including butnot limited to, dip coating, spray coating, precipitation coating, spraydrying, granulation, and the like.

In a further embodiment of the invention, a metallic phase may be coatedonto a template or shape forming substrate that then undergoescarburization and/or nitridation as previously detailed and described inthe embodiment involving silica.

After at least a portion of the metal oxide in the proppant material isreduced, the material can be subjected to nitriding or carbiding. Forinstance, the nitriding can use any nitrogen-containing material, suchas a nitrogen-containing gas. The nitrogen-containing gas can also serveas the fluidizing medium that replaces the previously present fluidizingmedium, such as the carbon monoxide or air. The nitriding can occur withany nitrogen-containing gas, for instance, pure nitrogen, anhydrousammonia gas, nitrogen oxides, or other gases. Optionally, hydrogen gasand/or helium gas can be additionally used with the nitrogen-containinggas.

The air that can be used as at least an initial fluidizing medium can bepre-heated, for instance, at a temperature of from about 25° C. to about1,000° C.

As an option, the method can further comprise introducing at least onecarbonaceous material before, during, and/or after the reducing agent ispresent. The carbonaceous material can be any carbonaceous materialsufficient to promote a catalytic or faster reaction so that the overallreduction of the metal oxide present in the proppant material occurs ata faster rate. Also, the carbonaceous material can alternatively oradditionally provide an additional source of carbon monoxide to serve asthe fluidizing medium and reducing agent. In the alternative or inaddition, the presence of carbonaceous material can provide a source ofcarbon to form a respective metal carbide in the proppant material. Thecarbonaceous material can be pulverized carbonaceous material. Thecarbonaceous material can be petroleum coke, charcoal, graphite, carbonblack, activated carbon, and the like. Further examples include anyother material reduced to elemental carbon or that can be reduced toelemental carbon, such as biomass materials that can include, but arenot limited to, cellulose materials, hardwood sawdusts, softwoodsawdusts, wood flour, starch, starch granules, potato granules,pulverized seeds, pulverized seed hulls, pulverized grasses and treewastes, pulverized coconut shells and/or fibers, pulverized rice hulls,pulverized grains such as wheat, corn, barley, oats, rye, buckwheat, andthe like. In the case of reducing the metal oxide to respective metal,the carbon:metal oxide ratio can be greater than 2 and preferably lessthan 6. From 2-6 parts of carbon to 1 part metal oxide by weight can beused. Other amounts can alternatively be used. In the case ofcarbonizing the respective metal to respective metal carbide, thecarbon:respective metal ratio can be greater than 1 and preferably lessthan 2. From 1-2 parts carbon to 1 part respective metal by weight canbe used. Other amounts can alternatively be used. The carbonaceousmaterial can be added to the metal oxide at the commencement of thereaction. The mass or amount of the carbonaceous material can beselected such that it meets the requirements of stoichiometry for thereaction and reacts completely with the metal phase to form the metalcarbide phase directly from the metal oxide phase. The mass or amount ofthe carbonaceous material can be added to the metal oxide phase at thecommencement of the reaction process in excess of that required forstoichiometry. The amount of carbonaceous material added in excess ofstoichiometry can range from 10 wt % to 100 wt % in excess ofstoichiometry, where the excess carbonaceous material reduces the metaloxide to the corresponding metal phase and carburizes the metal phase tothe metal carbide. The required mass or amount of the carbonaceousmaterial can be added to the reaction chamber with the metal oxide tocause the reduction of the metal oxide to the metal phase, and uponcompletion of the reduction reaction, further carbonaceous material canbe added to the reaction chamber to form the metal carbide phase.

In the case of nitriding the respective metal to the respectivemetal-nitride, the nitrogen:respective metal ratio can be greater than 2and preferably less than 5. From 2-5 parts nitrogen to 1 part respectivemetal by weight can be used. In the above ranges, the amounts of carbonor nitrogen used are based on elemental carbon or elemental nitrogencontent.

The carbonaceous material can be at least partially combusted during themethod. This partial combustion can lead to the carbon being convertedto carbon monoxide, thus supplementing the carbon monoxide used asfluidizing medium and reacting gas. An additional benefit to thiscombustion of the carbon in the furnace chamber allows a reduction ofthe heat input into the system from the external heating elements of thefurnace, and with careful control of furnace conditions and reactantflows, the system can become self-propagating and self-sustaining. As anoption, a secondary supply of air (or oxygen-containing gas, such asoxygen) may be introduced into the furnace chamber to effect combustionof a portion of the carbon to carbon monoxide. This air or oxygen can bepre-heated prior to being introduced into the reactor. Thus, prior toand/or during the reducing, an oxygen-containing gas can be introducedinto the reactor to partially combust the carbonaceous material (forinstance, that is present in excess of the stoichiometric requirementsof the reaction). As an option, at least a portion of the carbonaceousmaterial can react with at least a portion of the respective metal toform a respective metal-carbide phase.

In the present invention, the reducing step and/or nitriding step and/orcarbiding step can be repeated any number of times using the same ordifferent reducing agents, carbiding agents, and/or nitriding agentsand/or same or different conditions. The temperature, pressures, andtimes described above for silica can be used for this embodiment and canbe further adjusted depending on the metal oxide present in the proppantmaterial. The carbonaceous material can at least partially react withthe oxygen atoms in the metal oxide.

In a further embodiment of the present invention, a mixture (e.g.,homogenous) of micron or sub-micron sized metal carbide particles andthe corresponding metal particles of a micron or sub-micron size areadded to an aqueous and/or organic solvent carrier system. Polymericbased binders may also be added to the resulting ceramic/metal slurrysystem to improve the green strength of the dried mixture. The slurryformulation may be applied to a shape forming template via a spraycoating process to generate a shell with a specified thickness over thetemplate. The template may be either an inert material for example,cenospheres, envirospheres, and the like. Alternatively, the shapeforming template may be an organic material that pyrolyzes and thenthermally decomposes during heat treatment. Examples of such shapeforming templates include starch granules, polymer spheres, carbonspheres, and spheres from biomass materials such as wood pulp, woodflour, starch, rice hulls and the like. Following generation of theshell, the shell—template structure is heat treated in an inertatmosphere at a controlled heating rate. In addition, a carburizing gassuch as carbon monoxide may be added to the heat treatment atmosphere.During the heat treatment stage, the metallic particles contained withinthe shell material begin to melt and infiltrate the pores between themetal carbide particles, in a manner similar to the liquid phasesintering process as used for the densification of conventional oxideceramics. As the atmosphere is inert, the molten metal particles are notsubject to oxidation. The molten metal particles react with thecarburizing gas and form the corresponding metallic carbide phase, thusyielding a dense, fully sintered ceramic shell that may or may notcontain a minor percentage of residual metal. The use of an organicshape forming template is beneficial in that the carbon generated duringthe thermal decomposition of the organic material may serve to assistthe formation of the carbide phase.

In a further embodiment of the invention, a mixture (e.g., homogenous)of micron or sub-micron sized metal nitride particles and thecorresponding metal particles of a micron or sub-micron size are addedto an aqueous and/or organic solvent carrier system. Polymeric basedbinders may also be added to the resulting ceramic/metal slurry systemto improve the green strength of the dried mixture. The slurryformulation may be applied to a shape forming template via a spraycoating process to generate a shell with a specified thickness over thetemplate. The template may be either an inert material for example,cenospheres, envirospheres, and the like. Alternatively, the shapeforming template may be an organic material that pyrolyzes and thenthermally decomposes during heat treatment. Examples of such shapeforming templates include starch granules, polymer spheres, carbonspheres, and spheres from biomass materials such as wood pulp, woodflour, starch, rice hulls and the like. Following generation of theshell, the shell—template structure is heat treated in an inertatmosphere at a controlled heating rate. In addition, a nitriding gassuch as nitrogen, anhydrous ammonia, or nitrogen oxide may be added tothe heat treatment atmosphere. The nitriding gas may also be blendedwith hydrogen and/or helium. During the heat treatment stage, themetallic particles contained within the shell material begin to melt andinfiltrate the pores between the metal nitride particles, in a mannersimilar to the liquid phase sintering process as used for thedensification of conventional oxide ceramics. As the atmosphere isinert, the molten metal particles are not subject to oxidation. Themolten metal particles react with the nitriding gas component of theheat treatment atmosphere and form the corresponding metallic nitridephase, thus yielding a dense, fully sintered ceramic shell that may ormay not contain a minor percentage of residual metal. The use of anorganic shape forming template is beneficial in that the carbongenerated during the thermal decomposition of the organic material mayserve to form a carbide phase, thus yielding a composite structure ofthe metal nitride and metal carbide.

In a further embodiment of the present invention, a mixture (e.g.,homogenous) of micron or sub-micron metal carbide particles, micron orsub-micron metal particles, and organic binders that are suspended ineither an aqueous or organic carrier solvent may be formed into hollowspheres with a specified wall thickness and diameter using a coaxialnozzle method as taught previously. Upon separation of the sphere fromthe nozzle, the hollow sphere free falls through an organic solvent thatcontains a polymeric material that produces a thin coating over thewhole surface of the sphere. This polymeric coating serves to protectthe metallic particles from oxidation prior to the heat treatment stage.Following generation of the hollow spheres, the hollow spheres are heattreated in an inert atmosphere at a controlled heating rate. Inaddition, a carburizing gas such as carbon monoxide may be added to theheat treatment atmosphere. During the heat treatment stage, the metallicparticles contained within the shell material begin to melt andinfiltrate the pores between the metal carbide particles, in a mannersimilar to the liquid phase sintering process as used for thedensification of conventional oxide ceramics. As the atmosphere isinert, the molten metal particles are not subject to oxidation. Themolten metal particles react with the carburizing gas and form thecorresponding metallic carbide phase, thus yielding a dense, fullysintered ceramic shell that may or may not contain a minor percentage ofresidual metal.

In a further embodiment of the present invention, a mixture (e.g.,homogenous) of micron or sub-micron metal nitride particles, micron orsub-micron metal particles, and organic binders that are suspended ineither an aqueous or organic carrier solvent may be formed into hollowspheres with a specified wall thickness and diameter using a coaxialnozzle method as taught previously. Upon separation of the sphere fromthe nozzle, the hollow sphere free falls through an organic solvent thatcontains a polymeric material that produces a thin coating over thewhole surface of the sphere. This polymeric coating serves to protectthe metallic particles from oxidation prior to the heat treatment stage.Following generation of the hollow spheres, the hollow spheres are heattreated in an inert atmosphere at a controlled heating rate. Inaddition, a nitriding gas such as nitrogen, anhydrous ammonia, ornitrogen oxide may be added to the heat treatment atmosphere. Thenitriding gas may also be blended with a small percentage of hydrogenand/or helium. During the heat treatment stage, the metallic particlescontained within the shell material begin to melt and infiltrate thepores between the metal nitride particles, in a manner similar to theliquid phase sintering process as used for the densification ofconventional oxide ceramics. As the atmosphere is inert, the moltenmetal particles are not subject to oxidation. The molten metal particlesreact with the nitriding gas component of the heat treatment atmosphereand form the corresponding metallic nitride phase, thus yielding adense, fully sintered ceramic shell that may or may not contain a minorpercentage of residual metal.

In a further embodiment of the present invention, a mixture (e.g.,homogenous) of micron or sub-micron metal carbide particles, micron orsub-micron metal particles, and organic binders that are suspended ineither an aqueous or organic carrier solvent. The resultingceramic/metal slurry mixture is admitted to the chamber of a spray driervia a two-fluid nozzle. Control of the spray drier parameters (airflow,temperature, fluid flow, nozzle air, and the like) would allow theformation of droplet of slurry with controlled particle sizes to beformed. Further control of the slurry parameters, such as solidsloading, binder formulations, and rheology may allow the formation ofeither hollow or solid spheres of the ceramic powder. The spray driermay be operated in either co-current or counter-current mode withrespect the ceramic slurry flow and drying air flows in the system. Thespray drier may be of any design and the process air may be heated viaany means, including electrical heating elements, direct fired gasburners, or indirect fired gas burners. The capacity of the spray driermay be of any size, ranging from laboratory size, to pilot scale size toindustrial scale sizes. Drying of the droplets yields either solid orhollow spheres of the ceramic/metal mixture. Following formation of thespheres, the spheres are heat treated in an inert atmosphere at acontrolled heating rate. In addition, a carburizing gas such as carbonmonoxide may be added to the heat treatment atmosphere. During the heattreatment stage, the metallic particles contained within the shellmaterial begin to melt and infiltrate the pores between the metalcarbide particles, in a manner similar to the liquid phase sinteringprocess as used for the densification of conventional oxide ceramics. Asthe atmosphere is inert, the molten metal particles are not subject tooxidation. The molten metal particles react with the carburizing gas andform the corresponding metallic carbide phase, thus yielding a dense,fully sintered ceramic shell that may or may not contain a minorpercentage of residual metal.

In a further embodiment of the present invention, a mixture (e.g.,homogenous) of micron or sub-micron metal nitride particles, micron orsub-micron metal particles, and organic binders that are suspended ineither an aqueous or organic carrier solvent. The resultingceramic/metal slurry mixture is admitted to the chamber of a spray driervia a two-fluid nozzle. Control of the spray drier parameters (airflow,temperature, fluid flow, nozzle air, etc) would allow the formation ofdroplet of slurry with controlled particle sizes to be formed. Furthercontrol of the slurry parameters, such as solids loading, binderformulations, and rheology may allow the formation of either hollow orsolid spheres of the ceramic powder. The spray drier may be operated ineither co-current or counter-current mode with respect the ceramicslurry flow and drying air flows in the system. The spray drier may beof any design and the process air may be heated via any means, includingelectrical heating elements, direct fired gas burners, or indirect firedgas burners. The capacity of the spray drier may be of any size, rangingfrom laboratory size, to pilot scale size to industrial scale sizes.Drying of the droplets yields either solid or hollow spheres of theceramic/metal mixture. Following formation of the spheres, the spheresare heat treated in an inert atmosphere at a controlled heating rate. Inaddition, a nitriding gas such as nitrogen, anhydrous ammonia, ornitrogen oxide may be added to the heat treatment atmosphere. Thenitriding gas may also be blended with hydrogen and/or helium. Duringthe heat treatment stage, the metallic particles contained within theshell material begin to melt and infiltrate the pores between the metalnitride particles, in a manner similar to the liquid phase sinteringprocess as used for the densification of conventional oxide ceramics. Asthe atmosphere is inert, the molten metal particles are not subject tooxidation. The molten metal particles react with the nitriding gascomponent of the heat treatment atmosphere and form the correspondingmetallic nitride phase, thus yielding a dense, fully sintered ceramicshell that may or may not contain a minor percentage of residual metal.

In a further embodiment of the present invention, a mixture (e.g.,homogenous) of micron or sub-micron metal carbide powder, micron orsub-micron metal powder, and organic binder are added to the chamber ofa granulation device such as an Eirich mixer, a pan granulator, theBrunner granulator (as described in U.S. Pat. No. 3,690,622), or a spraygranulator. The powder mixture is granulated into solid spheres of aspecified diameter. Following the formation of the solid spheres, thespheres are heat treated in an inert atmosphere at a controlled heatingrate. In addition, a carburizing gas such as carbon monoxide may beadded to the heat treatment atmosphere. During the heat treatment stage,the metallic particles contained within the shell material begin to meltand infiltrate the pores between the metal carbide particles, in amanner similar to the liquid phase sintering process as used for thedensification of conventional oxide ceramics. As the atmosphere isinert, the molten metal particles are not subject to oxidation. Themolten metal particles react with the carburizing gas and form thecorresponding metallic carbide phase, thus yielding a dense, fullysintered ceramic shell that may or may not contain a minor percentage ofresidual metal.

In a further embodiment of the present invention, a mixture (e.g.,homogenous) of micron or sub-micron metal nitride powder, micron orsub-micron metal powder, and organic binder are added to the chamber ofa granulation device such as an Eirich mixer, a pan granulator, theBrunner granulator (as described in U.S. Pat. No. 3,690,622), or a spraygranulator. The powder mixture is granulated into solid spheres of aspecified diameter. Following the formation of the solid spheres, thespheres are heat treated in an inert atmosphere at a controlled heatingrate. In addition, a nitriding gas such as nitrogen, anhydrous ammonia,or nitrogen oxide may be added to the heat treatment atmosphere. Thenitriding gas may also be blended with hydrogen and/or helium. Duringthe heat treatment stage, the metallic particles contained within theshell material begin to melt and infiltrate the pores between the metalnitride particles, in a manner similar to the liquid phase sinteringprocess as used for the densification of conventional oxide ceramics. Asthe atmosphere is inert, the molten metal particles are not subject tooxidation. The molten metal particles react with the nitriding gascomponent of the heat treatment atmosphere and form the correspondingmetallic nitride phase, thus yielding a dense, fully sintered ceramicshell that may or may not contain a minor percentage of residual metal.

With respect to any of the methods involving the use of a mixture ofmetal carbide and/or metal nitride powders with at least one metalpowder, the particle size of the ceramic (metal) carbide or nitridepowder can range from 0.2 μm to 10 μm, or from 0.5 μm to 7 μm, or from 1μm to 5 μm, preferably from 0.3 μm to 1 μm. The particle size of themetal powder can range from 0.2 μm to 10 μm, or from 0.5 μm to 7 μm, orfrom 1 μm to 5 μm, preferably from 0.3 μm to 1 μm. The particle size ofthe metal powder may be the same or different to the particle size ofthe ceramic (metal) carbide or nitride powder. For any of the aboveparticle sizes, these numbers can be average particle size, or can bemaximum particles sizes. The methods of the present invention can makeproppants have controlled dimensions and/or controlled diameters. Withrespect to controlled dimensions and controlled diameter, the methods ofthe present invention can make proppant particle sizes having uniform ornearly uniform dimensions and/or diameters for a plurality of proppantparticles, meaning that the method provides a tight distribution in theproppants formed, such as where the proppant particles produced from themethod do not vary from the average dimensions and/or average diameterby more than 15%, or no more than 10% or no more than 5%, or no morethan 2% or no more than 1%, or no more than 0.75%, or no more than 0.5%,or no more than 0.1% or no more than 0.05%, or no more than 0.01% or nomore than 0.005%, or no more than 0.001%. As an option in any of theembodiment of the present invention, the proppant can have an outersurface and one or more methods of the present invention can furthercomprise treating the proppant to improve the hydrophobic nature of theproppant, the hydrophilic nature of the proppant, or to produce ahydro-neutral surface on the proppant. The manner in which this can beaccomplished can be done in the same manner as described below withrespect to the disclosure describing the surface modification on anoptional shell to achieve similar modification.

The present invention further relates to proppant products formed by theabove processes. Further, the present invention relates to a proppantthat comprises a metal-nitride phase(s), a metal-carbide phase(s), orboth. The proppant can optionally include a separate metal oxidephase(s), a separate respective metal phase or phases, and can containother phases. Other components can be present in the proppant, such asmetal components, metal oxide components, nitrides, carbides, and thelike. Examples of other materials that can additionally form theproppant of the present invention are described below with respect tothe template material described herein.

The proppant can be entirely or can contain silicon carbide, titaniumcarbide, boron carbide, zirconium carbide, aluminum carbide, tungstencarbide, molybdenum carbide, silicon nitride, titanium nitride, boronnitride, zirconium nitride, aluminum nitride, tungsten nitride, ormolybdenum nitride or any combination or mixture thereof. The proppantmay comprise a mixture of nitrides and carbides. The proppant can have ametallic-nitride phase or metallic-carbide phase or both. The proppantcan have a metallic-nitride phase present, but not a metallic-carbidephase. The proppant can have a metallic-carbide phase present, but not ametallic-nitride phase. The proppant may or may not contain a metallicoxide phase(s). The proppant may further have a metallic phase(s). Theproppant can have a metallic-nitride phase that is primarily present asthe outer surface of the proppant. The proppant can have ametallic-carbide phase that is primarily present as the outer surface ofthe proppant particle. The proppant can have a metallic-carbide phasethat is primarily present as the internal surface of the proppantparticle. As particular examples for the proppants of the presentinvention: the proppant can contain a nitride phase, carbide phase andresidual metallic phase; the proppant can contain a nitride phase andresidual metallic phase; the proppant can contain a carbide phase andresidual metallic phase; the proppant can contain only a carbide phase;the proppant can contain only a nitride phase; the proppant can have ametal phase may correspond to the carbide/nitride phase or may bedifferent; the proppant may be composed of boron carbide or siliconcarbide, or a mixture of these; the proppant may be composed of aluminumnitride, boron nitride or silicon nitride, or a mixture of these; theproppant may comprise boron carbide with or without boron nitride; theproppant may comprise boron carbide with or without silicon carbide; theproppant may comprise boron carbide with or without silicon nitride; theproppant may comprise boron carbide with or without aluminum nitride;the proppant may comprise boron nitride with or without silicon carbide;the proppant may comprise boron nitride with or without boron carbide;the proppant may comprise aluminum nitride with or without siliconcarbide; the proppant may comprise aluminum nitride with or withoutboron carbide; the proppant may comprise silicon nitride with or withoutsilicon carbide; the proppant may comprise silicon nitride with orwithout boron carbide; the proppant may contain a mixture of boroncarbide and any other carbide or nitride; the proppant may contain amixture of silicon carbide and any other carbide or nitride; theproppant may contain a mixture of boron nitride and any other carbide ornitride; the proppant may contain a mixture of aluminum nitride and anyother carbide or nitride; the proppant may contain a mixture of siliconnitride and any other carbide or nitride.

The proppant that has the metal-nitride phase(s) or the metal-carbidephase(s) or both can have one or both of these phases primarily on theexternal regions of the proppant material. These phases can be locatedprimarily on the surface of the proppant material. If the proppantmaterial is a sphere (or other similar shape) having a radius, as oneoption, the metal-carbide phase and/or metal-nitride phase can beprimarily located from the surface to half-way to the center of thesphere, from the surface to one-third of the radius or from the surfaceto one-fifth of the radius or from the surface to one-tenth of theradius or from the surface to two-thirds of the radius or from thesurface to 90% of the radius, and so on. The depth or degree of thenitride or carbide can range from 1% to 100% of the radius, and beuniform or non-uniform around the proppant surface area and/orinternally.

Other materials that form the proppant material can, in addition, beconverted to a nitride and/or carbide. For instance, other metal oxidesthat may form the proppant material can be converted to their respectivemetal and/or can be converted to their respective metal-nitride phase(s)or metal-carbide phase(s) or both. Thus, the various other componentsthat can be present, as described below for the template material, canbe in their current state as mentioned below, or can be converted totheir metal state, and/or can be converted to a nitride or carbide statebased on the methods of the present invention described above.

For purposes of the present invention, the below disclosure can beadditionally practiced with the present invention. In the discussionbelow, template materials and shells and methods of forming the two aredescribed. To the extent that any of the template materials containsilica or other materials that can be reduced, the template material canbe used in the methods of the present invention. Also, for purposes ofthe present invention, the various treatments to the template materialand/or substrate described below, the formation of one or more shellsand any other types of treatments described below can equally apply tothe proppant of the present invention. It is to be understood that thereference to template sphere, or substrate, does not necessitate that ashell be placed on the template sphere for purposes of the presentinvention. It is to be understood that the reference to templatematerial, template sphere, or substrate below would be a reference forpurposes of the present invention to the “proppant” or “proppantmaterial” described above in the method of making a proppant from aproppant material comprising a metal oxide. Thus, the various processingsteps, physical properties, sizes, amounts, and the like described belowwould equally apply to the embodiments described above with respect to aproppant containing a metal oxide using one or more methods of thepresent invention which involves the use of a reducing step and anitriding and/or carbiding step.

U.S. patent application Ser. No. 11/769,247 filed Jun. 27, 2007, U.S.patent application Ser. No. 11/728,953, filed Mar. 27, 2007, U.S. patentapplication Ser. No. 11/498,527, filed Aug. 3, 2006, U.S. patentapplication Ser. No. 11/347,664, filed Feb. 3, 2006, and U.S.Provisional Patent Application No. 60/649,594 filed Feb. 4, 2005, areall incorporated in their entirety by reference herein.

The materials of the present invention when used as proppants coulddominate current proppant solutions on all relevant quality dimensions.The methods of this invention are aimed at the fabrication of proppantsthat preferably exhibit neutral buoyancy, high crush strength, highsphericity, narrow size distribution, and/or high smoothness. Thesematerials have the ability to materially reduce and/or possiblyeliminate the need to employ expensive and reservoirpermeability-destroying polymer carrier gels.

Equally important, the optimal shape, size, size distribution, pore sizedistribution, and/or surface smoothness properties of the presentinvention suggest that flow resistance through the proppant pack couldbe reduced, such as by more than 50%. Neutral buoyancy enhances proppanttransport deep into the formation increasing the amount of fracture-areapropped thereby increasing the mechanical strength of the reservoir. Dueto the above issues, proppants of the present invention can achievesubstantially increased flow rates and/or enhanced hydrocarbon recovery.The low-cost of the present invention's preferred nanoparticles, and thereduced material requirements (on a per pound basis) are advantages ofthe present invention's preferred proppants. The low density of thepresent invention's proppants may enable reductions in transportationcosts in certain situations. Also, a lighter proppant allows moreproppant to be added, which can be useful in water-frac operations orother uses. Also, pumping costs can be lower because the proppant islighter and therefore less pumping force is needed which is helpful tocosts and does less damage to the formation since less pump pressure isused to pump the same volume of material. Further, significantlyimproved flow rate of the hydrocarbon recovery can occur.

The proppants of the present invention present oil and gas producerswith one or more of the following benefits: improved flow rates,improved productive life of wells, improved ability to design hydraulicfractures, and/or reduced environmental impact. The proppants of thepresent invention are designed to improve flow rates, eliminating ormaterially reducing the use of permeability destroying polymer gels,and/or reducing pressure drop through the proppant pack, and/or theability to reduce the amount of water trapped between proppants therebyincreasing hydrocarbon “flow area.”

The high density of conventional ceramic proppants and sands (roughly100 lb/cu.ft.) inhibit their transport inside fractures. High densitycauses proppants to “settle out” when pumped thereby minimizing theirefficacy. To maintain dense proppants in solution, expensive polymergels are typically mixed with the carrier solution (e.g. completionfluid). Once suspended in a gelled completion fluid, proppant transportis considerably enhanced.

Polymer gels are extremely difficult to de-cross link, however. As aresult, the gel becomes trapped downhole, coats the fracture, andthereby reduces reservoir permeability. Gel-related reservoirpermeability “damage factors” can range from 40% to more than 80%depending on formation type.

The neutral buoyancy property that can be exhibited by the proppants ofthe present invention preferably eliminates or greatly reduces the needto employ permeability destroying polymer gels as they naturally stay insuspension.

The proppants of the present invention are designed to improve reservoirflow rates by changing the hydrophilic properties of the proppantsthemselves. The hydrophilic nature of current proppants causes water tobe trapped in the pore spaces between proppants. If this water could beremoved, flow rates would be increased.

The use of extreme pressure, polymer gels, and/or exotic completionfluids to place ceramic proppants into formations adversely impacts themechanical strength of the reservoir and shortens its economic life.Proppants of the present invention preferably enable the use of simplercompletion fluids and possibly less (or slower) destructive pumping.Thus, reservoirs packed with neutrally buoyant proppants preferablyexhibit improved mechanical strength/permeability and thus increasedeconomic life.

More importantly, enhanced proppant transport enabled by neutralbuoyancy preferably enable the placement of the proppant of the presentinvention in areas that were heretofore impossible, or at least verydifficult to prop. As a result, the mechanical strength of the formationis preferably improved, preferably reducing decline rates over time.This benefit could be of significant importance—especially within “waterfracs” where the ability to place proppants is extremely limited.

If neutrally buoyant proppants are employed, water (fresh to heavybrines) may be used in place of more exotic completion fluids. The useof simpler completion fluids would reduce or eliminate the need toemploy de-crossing linking agents. Further, increased use ofenvironmentally friendly proppants may reduce the need to employ otherenvironmentally damaging completion techniques such as flashingformations with hydrochloric acid.

In addition to fresh water, salt water and brines, or synthetic fluidsare sometimes used in placing proppants to the desired locations. Theseare of particular importance for deep wells.

In the present invention a range of approaches for the synthesis andfabrication of proppants with designed buoyancy are disclosed. Theproppants are designed such that the material properties are such thatthe proppant preferably has neutral, positive, or negative buoyancy inthe medium chosen for pumping the proppant to its desired location inthe subterranean formation.

In the present invention, the proppant can be either solid throughout orhollow within the proppant to control buoyancy. In the presentinvention, a solid proppant is defined as an object that does notcontain a void space in the center, although a porous material would besuitable. A fully dense material is not a requirement of solid. A hollowmaterial is defined as an object that has at least one void space insidewith a defined size and shape.

In the present invention the proppant can be made from a ceramic, apolymer, or mixture thereof. The proppant can be made fromnanoparticles. The proppant can be a composite or combination ofceramic, polymer and other materials. Although not required it isunderstood that a ceramic may include oxides such as aluminum oxides(alumina) or mixed metal aluminum oxides (aluminates).

The strength properties for a proppant can be dependent on theapplication. It is intended that a crush strength of 4000 psi to 12,000psi or higher is desirable. However, for specific applications,crush-strengths of greater than 9000 psi or greater than 12000 psi aredesirable. Other crush strengths below or above these ranges arepossible.

The optimum size of the proppant can also be dependent on the particularapplication. Part of the present invention is that it is possible todesign various proppant sizes. Sizes (e.g., particle diameters) may varyfrom 10 μm to 10,000 μm. The particle diameter can be in the range offrom 50 μm to 2,000 μm.

The proppant can be made from a single-phase material or can be madefrom a multi-phase system, such as from a two phase system thatcomprises a substrate (or template) and a second phase. A summary ofexemplary templates and substrates is shown in Table 3. The substrate ortemplate may be an inorganic material such as a ceramic or glass.Specifically, the ceramic can be an oxide such as aluminum oxides(alumina) as well as mixed metal aluminum oxides such as metalaluminates containing calcium, yttrium, titanium, lanthanum, barium,and/or silicon in addition to aluminum. In order to make variablebuoyant proppants, it is preferable to use a ceramic cenosphere orsimilar glass-like hollow sphere as the substrate or template. Thesubstrate or template (or hollow material useful in the shell) can beobtained from such sources as Macrolite by Kinetico, Noblite by NobliteMicrospheres, Omega Bubbles by Omega Minerals, or Poraver by DennertPoraver GmbH.

Alternatively, the substrate or template may be an organic material,such as a polymer or organic molecule or surfactant. Although notlimited as such, the polymer may be chosen from polystyrene, latex,polybutadiene, polyethylene, polypropylene and chemically relatedpolymers. The polymer can be a naturally occurring material such as apeptide or protein.

Alternatively, the substrate can be a naturally occurring materialchosen from plant or tree material, such as plant seeds, crushed nuts,whole nuts, plant pips, cells, coffee grinds, or food products.

In a two-phase system, the second phase can coat the supporting ortemplate first phase, or infiltrates the supporting or template firstphase, or reacts with the supporting or template first phase.

The second phase can be a polymer, such as an epoxide, polyolefin, orpolymethacrylate. Furthermore, a nanoparticle material such as analumoxane optionally containing chemical functional groups that allowfor reaction with the polymer can reinforce the polymer. Suitablechemical functional groups or substituents include, but are not limitedto, hydroxides, amines, carboxylates, or olefins.

The second phase can also be a ceramic or glass. The ceramic can be anoxide, such as aluminum oxide called alumina, or a mixed metal oxide ofaluminum called an aluminate, a silicate, or an aluminosilicate, such asmullite or cordierite. The aluminate or ceramic may contain magnesium,calcium, yttrium, titanium, lanthanum, barium, and/or silicon. Theceramic may be formed from a nanoparticle precursor such as analumoxane. Alumoxanes can be chemically functionalized aluminum oxidenanoparticles with surface groups including those derived fromcarboxylic acids such as acetate, methoxyacetate, methoxyethoxyacetate,methoxyethoxyethoxyacetate, lysine, and stearate, and the like.

The designed proppant can be suspended in a liquid phase. The liquidphase may make the proppant more easy to transport to a drill site.Transportation may be by rail transport, road or ship, or any otherappropriate method, depending on geography and economic conditions. Inaddition to transport to the drill site, the suspended mixture ispreferably pumpable or otherwise transportable down the well to asubterranean formation and placed such as to allow the flow ofhydrocarbons out of the formation.

Specific methods for designing proppants with specific buoyancy,strength, size, and/or other desirable properties are summarized below.

A proppant particle with controlled buoyancy and crush strength used toprop open subterranean formation fractures can be made from a naturallyoccurring substrate coated with an organic polymer or resin coatingpreferably containing a nanoparticle reinforcement. The naturallyoccurring substrate can be chosen from the following group: crushed nutshells, plant seeds, coffee grinds, plant pips, or other food products.The organic polymer or resin can be chosen from the following group:epoxide resin, polyethylene, polystyrene, or polyaramide. Thenanoparticle reinforcement can be of various types, but is preferably acarboxylate alumoxane in which the carboxylate alumoxane optionally hasone or more types of chemical functional groups that can react orotherwise interact with the polymer resin and/or also allow for thealumoxane to be miscible with the polymer. Proppants of this design maybe made by suspending a substrate material in a suitable solvent, addingthe polymer, resin or resin components, adding the nanoparticle,allowing the resin and nanoparticle mixture to coat the substratematerial, and drying the coated particle. The nanoparticle and resincomponents can be pre-mixed before addition to the substrate, and asolvent or other components can be a component of the resin or polymer.

A proppant particle with controlled buoyancy and crush strength used toprop open subterranean formation fractures can be made from a ceramicsubstrate and an organic polymer or resin coating. The ceramic substrateor template can be a non-porous or porous particle and can be a solid orhollow particle. It is preferable that the particle is a hollowspherical particle such as a cenosphere or similar product. Cenospherescan be commercially produced ceramic or glass hollow spheres that aremade as side products in various industrial processes. The organicpolymer or resin can be chosen from the following group: epoxide resin,polyethylene, polystyrene, or polyaramide. Proppants of this design maybe made by suspending a substrate material in a suitable solvent, addingthe polymer, resin or resin components, allowing the resin to coat thesubstrate material, and drying the coated particle. It is possible touse a solvent to facilitate the coating process. An improved version ofthis proppant can be prepared by the addition of nanoparticles forreinforcement, such as an alumoxane optionally with chemical functionalgroups that react and/or allow miscibility with the polymer resin. Analternative method of controlling the properties of the proppant is toadd a linker group to the surface of the ceramic substrate that canreact with the organic polymer or resin coating.

A reinforced template sphere can be formed by providing a templatesphere and reinforcing around the outer surface of said template sphereby direct nucleation and deposition of species onto the template sphere.The direct nucleation and deposition can comprise a precipitationreaction. The species can be any species mentioned in this invention.Precursors for precipitation include, but are not limited to: titaniumtetrachloride, titanium oxychloride, titanium chloride, titaniumoxysulphate, titanium sulphate, and titanium organo-metallic compoundssuch as titanium isopropoxide, calcium chloride, calcium hydroxide,aluminum chloride, aluminum nitrate, aluminum sulphate, and aluminumorgano-metallics for example aluminum isopropoxide, and the like.

A proppant particle with controlled buoyancy and/or crush strength usedto prop open subterranean formation fractures can be made from a ceramicsubstrate, a ceramic coating, or infiltration. The ceramic substrate ortemplate is preferably spherical and hollow such as a cenosphere orsimilar material. However, any suitable substrate that provides theresulting properties of the proppant may be used. The ceramic coating orinfiltration can be an oxide, for instance, an oxide of aluminum or amixed metal oxide of aluminum. A proppant of this type may be preparedby coating of a spherical template with a ceramic precursor solution,drying the coated ceramic particle, and heating the coated ceramicparticle to a temperature sufficient to form ceramic sphere of desiredporosity and hardness. The ceramic precursor may be a nanoparticle suchas an alumoxane, or a sol-gel precursor. Proppants of this type may beprepared by suspending the ceramic substrate in a suitable solvent,adding a ceramic precursor, allowing the ceramic precursor to coat theceramic substrate, drying the coated ceramic particle, and heating thecoated ceramic particle to a temperature sufficient to form ceramicspheres of desired porosity and hardness.

TABLE 3 Possible Templates for proppants Food and food Plants andproducts minerals Waste Other Coffee, Soils, Slag (steel, coke) a)Polypropylene, Milk, Bauxite, Silicas b) Glass Beads Whey, Cellulose,(diatomaceous earth, c) Surfactants/ Animal/Fish Guargum, diatomite,kesselgur, Detergents Eggs, Algae, Popped Perlite, d) Polystyrene Nuts,Lignin, Vermiculate), e) Bacteria Small corn Poppy Seeds, Fly ash(coke), f) Erasers pieces. Mustard Seeds Rust, g) Soap Sawdust Woodflour, Rape Seeds Gypsum h) Macrolite Grain Husks Plankton Pieces(fertilizer), (corn, maize, of sponge Rubber (tires), mailo, capher,Fur/Hair, Kohl Spent FCC sorgum). Rabi Seeds catalyst, Spent Motor OilUsed adsorbents Flue gas filter cakes from bags in baghouses cenospheres

In the present invention, the invention, in part, relates to a proppantused to prop open subterranean formation fractions comprising a particleor particles with controlled buoyancy and/or crush strength. Thecontrolled buoyancy can be a negative buoyancy, a neutral buoyancy, or apositive buoyancy in the medium chosen for pumping the proppant to itsdesired location in the subterranean formation. The medium chosen forpumping the proppant can be any desired medium capable of transportingthe proppant to its desired location including, but not limited to a gasand/or liquid, energized fluid, foam, like aqueous solutions, such aswater, brine solutions, and/or synthetic solutions. Any of the proppantsof the present invention can have a crush strength sufficient forserving as a proppant to prop open subterranean formation fractures. Forinstance, the crush strength can be 1,000 psi or greater, 3,000 psi orgreater, greater than 4000 psi, greater than 9000 psi, or greater than12000 psi. Suitable crush strength ranges can be from about 3000 psi toabout 15000 psi, or from about 5000 psi to about 15000 psi, and thelike. In some applications, like coal bed methane recovery, a crushstrength below 3000 psi can be useful, such as 500 psi to 3000 psi, or1000 psi to 2,000 psi.

The proppants of the present invention can comprise a single particle ormultiple particles and can be a solid, partially hollow, or completelyhollow in the interior of the particle. The particle can be spherical,nearly spherical, oblong in shape (or any combination thereof) or haveother shapes suitable for purposes of being a proppant.

The proppant can have any particle size. For instance, the proppant canhave a particle diameter size of from about 1 nm to 1 cm or a diameterin the range of from about 1 micron to about 1 mm, or a diameter of fromabout 10 microns to about 10000 microns, or a diameter of from about1000 microns to about 2000 microns. Other particle sizes can be used.Further, the particle sizes as measured by their diameter can be abovethe numerical ranges provided herein or below the numerical rangesprovided herein.

The particle comprising the proppant can be or can contain a ceramicmaterial. The ceramic material can comprise an oxide such as an oxide ofaluminum. The ceramic material can comprise an aluminate. For instance,the aluminate can be an aluminate of calcium, yttrium, titanium,lanthanum, barium, or silicon, or any combinations thereof, and/or otherelements that can form aluminates.

In the present invention, the particle(s) forming the proppant cancomprise a substrate or template and a second phase, such as a coatingon the substrate or template. The substrate or template can be a polymeror surfactant or ceramic material. The polymer, for instance, can be anythermoplastic or thermoset polymer, or naturally occurring polymer. Forinstance, the polymer can be a polystyrene, a latex, or polyalkylene,such as a polyethylene or polypropylene. The polymer can be apolybutadiene, or related polymers or derivatives of any of thesepolymers. The polymer can be a naturally occurring material or cancontain a naturally occurring material. For instance, the naturallyoccurring material can be a peptide or protein, or both.

With respect to the substrate or template, the substrate or template canbe a naturally occurring material or can contain a naturally occurringmaterial. For instance, the naturally occurring material can be a plantmaterial or tree material. For instance, the naturally occurringmaterial can be a plant seed, a crushed nut, whole nut, plant pip, cell,coffee grind, or food products, or any combination thereof. The ceramicmaterial can comprise a cenosphere.

The second phase or coating, or shell can coat the template orsubstrate. The second phase or template can infiltrate the template orsubstrate. Further, or in the alternative, the second phase or shell orcoating can react with the substrate or template, or a portion thereof.

The second phase, coating, or shell, can comprise one or more polymerssuch as a thermoplastic or thermoset polymer(s). Examples include, butare not limited to, an oxide, polyolefin, polymethacrylate, and thelike. The coating, shell, or second phase can optionally be reinforcedby nanoparticles. The nanoparticle material can be any type of materialcapable of acting as a reinforcement material. Examples include, but arenot limited to, ceramics, oxides, and the like. Specific examplesinclude, but are not limited to, alumoxane. The alumoxane can optionallycontain one or more chemical functional groups that are on thealumoxane. These chemical functional groups can permit, facilitate, orotherwise permit reaction with a polymer that also forms the coating orshell, or the polymer that may be present in the template or substrate.Examples of substituents that may be on the nanoparticles, such as thealumoxane, include, but are not limited to, hydroxides, amines,carboxylates, olefins, and/or other reactive groups, such as alkylgroups, aromatic groups, and the like.

The coating or shell or second phase can be or contain a ceramicmaterial(s), such as an oxide(s). Specific examples include, but are notlimited to, an oxide(s) of aluminum. The ceramic can be an aluminate oralumina. For instance, the aluminate can be an aluminate of calcium,yttrium, titanium, lanthanum, barium, silicon, or any combinationthereof, or can contain other elements. The material forming the coatingor shell can be initially in the form of a nanoparticle such as analumoxane. The alumoxane can be acetate, methoxyacetate,methoxyethoxyacetate, methoxyethoxyethoxyacetate, lysine, stearate, orany combination thereof.

The proppant can be suspended in a suitable gas, foam, energized fluid,or liquid phase. The carrier material, such as a liquid phase isgenerally one that permits transport to a location for use, such as awell site or subterranean formation. For instance, the subterraneanformation can be one where proppants are used to improve or contributeto the flow of hydrocarbons, natural gas, or other raw materials out ofthe subterranean formation. In another embodiment of the presentinvention, the present invention relates to a well site or subterraneanformation containing one or more proppants of the present invention.

The proppant which preferably has controlled buoyancy and/or crushstrength can have a naturally occurring substrate or template with anorganic polymer or resin coating on the template or substrate andwherein the organic polymer or resin coating contains nanoparticles,preferably for reinforcement purposes. As specific examples, butnon-limiting examples, the naturally occurring substrate can be acrushed nut, cell, plant seed, coffee grind, plant tip, or food product.The organic polymer or resin coating, for instance, can be an epoxyresin, polyethylene, polystyrene, or polyaramide, or other thermoplasticor thermoset polymers. The nanoparticle can be an alumoxane, such as ancarboxylate alumoxane or other ceramic material. The alumoxane can haveone or more chemical functional groups that are capable of reacting withthe organic polymer or resin coating. The functional groups canoptionally allow the ceramic materials, such as alumoxane, to bemiscible with the polymer or resin coating. The crush strength of thisproppant can be as described earlier. The proppant can have a diameteras described earlier or can be a diameter in the size range of fromabout 25 to about 2000 microns. Other diameter size ranges are possible.

In the present invention, the template or substrate can be a ceramicmaterial with an organic polymer or resin coating. The ceramic substratecan be a porous particle, substantially non-porous particle, or anon-porous particle. The ceramic template or substrate can be spherical.The ceramic substrate can be a hollow particle. For instance, theceramic substrate can be a cenosphere. The organic polymer or resincoating can be as described above. The crush strength can be the same asdescribed above. The diameter can be the same as described earlier.Optionally, the proppant can have nanoparticles for reinforcement valueor other reasons. The nanoparticle can be in the polymer or resincoating. The nanoparticle can be the same as described earlier.

The proppant can have a substrate or template containing or made fromone or more ceramic material(s). A linker group can be located on thetemplate or substrate. A shell or coating containing a polymercontaining a resin coating can be located around this template orsubstrate having the linker group. More than one type of linker groupcan be used. The linker group, in at least one embodiment, permitsbonding between the substrate or template and the coating. The linkergroup can be a coupling agent. The coupling agent can be of the typeused with metal oxides.

The proppant can have a substrate or template that comprises a ceramicmaterial and further has a coating or shell that comprises a ceramicmaterial that can be the same or different from the template material.The template or substrate and the shell or coating can have the samecharacteristics and parameters as described above for the otherembodiments, such as shape, crush strength, buoyancy, and the like.Preferably, the ceramic substrate or template is a cenosphere and theceramic coating or shell is an oxide, such as an oxide of aluminum oraluminate, a silicate, or an aluminosilicate. Other examples include,but are not limited to, shells that contain silicon, yttrium, magnesium,titanium, calcium, or any combinations thereof.

The proppants of the present invention can be made a number of ways. Forinstance, the substrate material can be suspended in a suitable solventand then the material forming the shell or coating can be added to thesolvent containing the suspended substrate material. Optionally,nanoparticles can be added. The coating material, such as the polymer orresin, as well as the nanoparticle(s) present as a mixture can then coatthe substrate material. Afterwards, the coated particle is dried usingconventional drying techniques such as an oven or the like. The optionalpresence of nanoparticles can optional react with the coating material,such as the polymer or resin. Furthermore, if nanoparticles are used,the nanoparticles can be added separately or can be pre-mixed with thecoating components, such as the resin or polymer, prior to beingintroduced to the suspension of substrate material. The solvent that isused to suspend the substrate material can be part of or present withthe polymer or resin coating materials. Furthermore, the coatingmaterials can optionally cross link during the coating process toperform a cross-linked coating on the substrate or template.

As another option, if a linkage molecule or material is used, thelinkage molecule can be reacted with the substrate or template prior tobeing suspended in a solvent, or after the substrate material issuspended in a solvent. The linkage molecule optionally reacts with thesubstrate or template material and optionally reacts with the coating orshell material such as the resin or polymer. Again, nanoparticles can beadded at any point of this process.

In another method of making one or more types of proppants of thepresent invention, a template or substrate material can be coated, suchas with a precursor solution such as a ceramic containing precursorsolution. The coated template material can then be dried andsubsequently heated to a temperature to form a densified shell, forinstance, having desirable porosity or hardness, or both. Preferably,the material is in the shape of a sphere. In this embodiment, theprecursor solution preferably comprises nanoparticles such as ceramicnanoparticles. For instance, ceramic particles can be alumoxane. Theprecursor solution can be in the form of a sol-gel. For instance, thesol-gel can contain aluminum as well as other elements. The template orsubstrate can be a hollow particle and/or can be spherical in shape. Thecoating that coats the ceramic template can optionally react with thesubstrate, for instance, during the heating step.

The substrate or template, preferably prior to the optional second phaseor optional coating or optional shell being present, can be treated inone or more ways to remove or diminish flaws on the surface of thesubstrate or template. These flaws may be convex or concave in nature orboth. This can be especially beneficial when the template or substrateis an inorganic material. The removal or diminishing of these flaws,especially strength-limiting flaws, can permit the second phase orcoating or shell to provide more enhanced strengthening of the substrateor template and the overall proppant. The flaws can include, but are notlimited to, peaks, protuberances, ridges, craters, and other flaws whichcan include surface undulations, which are significantly different fromthe overall surface texture or surface smoothness of the substrate ortemplate. In one or more embodiments, a flaw in a material system can beconsidered as anything which negatively impacts the apparent strength ofa material system. Flaws can be physical and/or chemical in nature.Physical flaws may include such things as protuberances, bumps,scratches, grooves, pores, pits, dislocations, and/or defects in thecrystal structure of the material. Chemical flaws may include phasesthat prevent solid state bonding, e.g. grain boundary phases,modification of the crystal structure through atomic substitution, andthe like By removing or diminishing these flaws in the surface of thesubstrate or template, the sharp protuberances or peaks or ridges can beremoved or diminished, and a surface can be created that is “smootherand more spherical” which can permit the second phase or coating orshell to be more effective in providing strength to the overallproppant. There are one or more ways to quantitatively show the removalor diminishing of flaws on the surface of the substrate or template. Forinstance, an aspect ratio (AR) test can be used and/or the radius ofcurvature (RC) can be compared.

In the aspect ratio test, representative micrographs are taken of anumber of spheres (at least 40). For purposes of the test, 40 particlesare studied and the results are averaged together. The size of theprotuberances, bumps, etc (also called artifacts) on the surface of thespheres are measured. Two measurements of each artifact are taken. Thefirst measurement, x₁ being taken radially from the base of the artifactto the tip and the second measurement, x₂ taken at the full width athalf maximum point (FWHM) of the artifact in a perpendicular directionto the first, as shown in FIG. 12.

The aspect ratio, AR is calculated by dividing the first measurement bythe second measurement (i.e. x₁/x₂) to obtain an aspect ratio for theartifact. As the aspect ratio approaches unity, the morphology of theartifact becomes spherical. The effectiveness of the treatment of thetemplates can be determined by the reduction in the aspect ratio of theartifacts on the surface.

For purposes of the present invention, the AR can be 5 or less, such asfrom 0.1 to 5, or from 0.2 to 5, or from 0.5 to 5, or from 0.1 to 4, orfrom 0.1 to 3, or from 0.1 to 2, or from 0.8 to 1, or from 0.1 to 1.8,or from 0.1 to 1.5, or from 0.5 to 1.3, or from 0.7 to 1.2 and otherranges.

In the radius of curvature (RC) test, representative micrographs (atleast 40) are taken. Again, for purposes of the test, 40 particles arestudied and the results are averaged together. The length of the curvedsurface over an angular range of 180° (π radians) of the artifact ismeasured, denoted by s in FIG. 13. For purposes of the RC test, themiddle of the peak or flaw is identified and then 10% on each side ofthe peak tip or flaw is determined and the length of this curve ismeasured. The % is based on the total distance from the peak or middleof the flaw to the template surface. From the value of the surfacelength, the radius of the curvature, r can be determined from thefollowing expression:

r=s/π.

The RC can typically range from 0.5 μm to 100 μm, with the upper limitof the radius of curvature being the radius of the template.

Also, when the flaws or artifacts are removed or reduced, the overallsurface area (m²/g) of the template can be reduced, such as a reductionin surface area of 1% to 10% or more compared to the template prior tosurface treatment.

As some specific examples, templates were measured before and aftersurface treatment with respect to each test, AR and RC and using one ormore of the surface treatments achieved a reduction of flaws orartifacts.

Test #1

Pre-treatment of template Post-treatment of template x₁ = 10 μm x₁ = 4μm x₂ = 1 μm x₂ = 2 μm AR = 10/1 = 10 AR = 4/2 = 2

Test #2

Pre-treatment of template Post-treatment of template s = 5 μm s = 13 μmr = 5/π ≈ 1.6 μm r = 13/π ≈ 4.1 μm

The treatments to remove or diminish flaws on the substrate or templatecan include one or more chemical treatments, one or more mechanicaltreatments, and/or one or more thermal treatments, and/or sparkdischarge treatment, or any combination of treatments.

For example, the chemical treatment can include the use of any chemical(e.g., compound) in the form of a solid, gas, or liquid which willdissolve or otherwise react with one or more parts of the surface of thesubstrate or template to remove or reduce the flaws described above. Thechemical treatment of the template or substrate may include preferentialdissolution of one or more species contained in and/or on the templatestructure, for example the preferential dissolution of silica from thetemplate material to reduce the overall silica content (e.g., reducefrom 1% to 99% or from 10% to 90%, or from 20% to 70% be weight of theone or more species as originally present on the surface). Examples ofsuch chemicals include, but are not limited to, sodium hydroxide,potassium hydroxide, calcium carbonate, calcium hydroxide, acids, suchas phosphoric acid, orthophosphoric acid, nitric acid, hydrofluoricacid, and the like. The chemical treatment of the templates can occurfor any time, such as various reaction times, such as from 1 minute to750 minutes, or from 5 minutes to 500 minutes, or any other time rangeswithin these ranges or outside of these ranges. Chemical treatment canoccur in any reaction container or vessel, such as an agitated tankcontaining the reagent(s). The amount of the reagent or chemical usedfor treatment can vary depending upon the amount of substrate ortemplate present and the number of surface flaws being reduced orremoved. As an example, the amount of the reagent or chemical can befrom about 10 wt % to about 70 wt %, such as from about 10 wt % to about40 wt %. After treatment, the reagent or chemical can be removed usingstandard techniques, such as a filter, and also using standard washingtechniques, such as spraying with water. In addition to the reduction ofsilica, the method (or other methods described herein) may also be usedto reduce other contaminant phases present in the template system, forexample iron oxides, free carbon, calcium oxides, sodium oxides, and/orvarious other contaminant phases that are formed during synthesis of thetemplate system.

The treatment, such as the chemical treatment, may also be to activate,passivate, and/or alter the physical properties of the template surface,such as inducing or improving the hydrophilic, hydrophobic orhydroneutrality of the surface. Examples include the use of chemicalspecies that induce crystallization of the free amorphous ornon-crystalline silica of the template. For example, the use of titaniumoxide based treatments can induce the crystallization of the silica atreduced temperatures, and thus reduce the silica's propensity toinfiltrate and reduce grain boundary strengths.

As another example, mechanical treatment(s) can be used, as indicated,to remove or reduce flaws on the substrate or template. The mechanicalmethods can include, but are not limited to, tumbling, tumbling in thepresence of abrasive material, impingement, such as high-speed elasticcollisions (impingement) with rigid surfaces, and the like. For example,tumbling of the substrate or templates alone can result in the fractureand consequent elimination of surface flaws. Tumbling of the substratesor templates in the presence of abrasive material can result in thegradual erosion of the surface flaws. Examples of abrasive material ormedia include, but are not limited to, metal oxides, carbides, and thelike. Specific examples include, but are not limited to, aluminum oxide,silicon carbide, zirconium oxide, cerium oxide, iron oxide, or anycombination thereof. Impingement can include high-speed impingement, forinstance, on rigid surfaces, which can result in impact fracture and,therefore, the removal or reducing of the surface flaws. The amount ofabrasive material used can be from about 5 wt % to about 50 wt %, suchas from about 10 wt % to about 30 wt %. The abrasive media can have asize of from 1 μm to 500 μm and more preferably from 10 μm to 100 μm.The abrasive media, after being used, can be separated through standardseparation techniques, such as, but not limited to, screening/sieving,sedimentation, and flotation. The high-speed impingement can occur atspeeds of from 5 ms⁻¹ to 100 ms⁻¹, such as 10 ms⁻¹ to 30 ms⁻¹, and canbe achieved by feeding the substrate or templates into a vortex streamor other stream which can yield particle-to-particle and/orparticle-to-wall interactions sufficient to result in the removal orreduction of one or more surface flaws as explained above. The tumblingof the substrate or template can be achieved in standard equipment usedfor tumbling, such as a ball mill or other closed vessel which rotatesabout its longitudal axis.

With respect to a thermal treatment to remove or reduce surface flaws onthe substrate or template, various devices and methods can be used whichprovide sufficient temperature to cause the reduction or removal of oneor more surface flaws. These devices and/or methods include, but are notlimited to, static bed furnaces, dynamic bed furnaces (e.g., rotatingtube furnaces), fluidized bed furnaces, direct injection into hightemperature flames (for instance, oxidizing flames), use of a gas flamecontained within a combustion tube mounted vertically and/or injectioninto a high temperature plasma flame. The residence time for thisthermal treatment can be any time sufficient to reduce or remove surfaceflaws, such as from 100 milliseconds to 200 minutes, and the residencetime generally is lower for the direct injection into oxidizing flame orplasma flame techniques. The temperature achieved during thermaltreatment can be any temperature sufficient to remove or reduce surfaceflaws, such as a temperature of 600° C. to 1,000° C., or greater than1,000° C., such as from 1,000° C. to 1,500° C. or even higher, such asfrom 1,500° C. to 5,000° C. or higher. Generally, the temperature rangeof 1,500° C. or higher is more suitable for the flame injection methodand the temperature of 5,000° C. or higher is more obtainable for plasmaflame injection methods. During the thermal treatment, pressure can alsobe used to control the template structure or the physical properties ofthe template. For instance, pressures may be used from less than 0.1 atmto 10 atm or greater, such as from 1 atm to 5 atm. When the localizedpressure is reduced to values below that of ambient pressure, this canlead to the expansion of the template and consequently a reduction inthe density of the template. When the localized pressure is increasedabove ambient pressure, this can result in the contraction of thetemplate to smaller overall diameters and hence an increase in thedensity of the template. Thus, this technique provides the ability to“dial in” or control certain densities of the proppant material. Also,the reduction or increase in pressure can improve the morphology of thetemplate, especially when the template is held or is very near the glasstransition temperature of the template material. The change in localizedpressure can be achieved through the use of various pressure controldevices and techniques, such as a sealed ceramic tube surrounded byheating elements and containing the template material, wherein theceramic tube is connected to either a pressure source or sink.Pressurizing fluid can be used to achieve the desired localizedpressure, such as air, nitrogen, inert gases, such as argon, or any gas.

The thermal treatment has the ability to combust “free carbon”particulates and/or other impurities from the template surface. Further,the thermal treatment has the ability to anneal the template surface toreduce potentially harmful strain fields. The thermal treatment has theability to preferentially melt or otherwise remove or reduce the surfaceflaws, especially the “sharp” protuberances on the surface of thetemplate due to the high radius of curvature.

During thermal treatment, it is an option to include an active phase inorder to react with potentially harmful phases in the template material,such as amorphous silica. The addition of active phase material caninclude, but is not limited to, boehmite, alumina, spinel,alumnosilicate clays (e.g., kaolin, montmorillonite, bentonite, and thelike), calcium carbonate, calcium oxide, magnesium oxide, magnesiumcarbonate, cordierite, spinel, spodumene, steatite, a silicate, asubstituted alumino silicate clay or any combination thereof (e.g.kyanite) and the like. Thermal treatment may also be carried out inalternate atmospheres, other than that of air to induce the formation ofnew phases, for example thermal treatment in a nitriding atmosphere toform the nitrides, or thermal treatment in a carburizing atmosphere toform the carbides. Examples of such atmospheres include carbon monoxide,nitrogen, nitrogen oxide, nitrogen dioxide, dinitrogen pentoxide,anhydrous ammonia, and the like.

The thermal treatment of the template or substrate may include suchprocesses as annealing, quenching, and/or tempering, or a combinationthereof. The thermal treatment may also include the pyrolysis orcombustion of volatile phases contained in the structure of the templateor substrate material. The thermal treatment of the template orsubstrate material may be conducted using either a static or a dynamicsystem. The thermal and chemical treatments may be combined in anyfashion, such as into a single process to achieve a multiplicity ofresults, for example the tumbling action afforded by a rotating hearthfurnace may be utilized to achieve a chemical reaction between thetemplate and an active phase as well as provide an overall improvementin the morphology of the template material.

With respect to the use of a gas flame contained within a combustiontube mounted vertically, the template can be introduced above the flamevia feed tubes located on the periphery of the combustion tube and underthe influence of gravity pass into the hot zone above the flame.Fluidization of the templates via the flame induced updraft andturbulence causes improved spheronization of the templates and reductionof flaws on the surfaces of the templates. The treated templates maythen be removed from the combustion tube via a negative pressure appliedto the top section of the combustion tube or via gravity at the bottomof the combustion tube in the annular space around the burner or someother manner. This method may also be used to sinter the ceramic shellof the proppant after coating. The furnaces that are used for theexpansion of perlite ore, can be used for this embodiment.

In one example, thermal treatment of templates at a temperature of 700°C. for 120 minutes prior to coating has provided a strength increase ofthe proppant on the order of 1,000 psi, compared to the same proppantnot being thermally treatment prior to receiving the coating or shell.

Another treatment of the template can be achieved through a sparkdischarge treatment, wherein the template or substrate material can besuspended in a fluidizing air stream or a dielectric fluid, such as in aconductive vessel which is electrically grounded. The surfaces of eachtemplate can be electrically charged to a high enough potentialdifference with respect to the ground to induce a corona-type discharge.This corona discharge occurs when the potential difference is highenough and initiates from the template surface at the areas of lowestradius of curvature to the walls of the vessel. This results inlocalized extreme heating of these areas leading to melting andevaporation at that point, thus removing a peak, protuberance, or ridge,or surface flaws described above. The charge may be applied to thetemplates in any fashion.

The charge may be applied to the templates by way of a van de Graffgenerator, Tesla coil, or electron beam impingement. Radio frequency(RF), or induction coupling can be used to induce eddy currents in thesurface regions of the templates and thus generate substantial surfacecharge on the templates. As indicated, the benefit of such a methodwould be to remove the “sharp” protuberances that have a negative effecton the mechanical properties of the proppant system.

The properties of the template or substrate may be modified by way ofpressure and/or temperature effects. A variation in pressure with theapplication of thermal energy maybe used to alter the specific gravityof the template material.

The proppant can be obtained by suspending a substrate, such as aceramic substrate in a suitable solvent such as water or other aqueoussolutions. The ceramic precursor which coats the template or substratecan be added. The ceramic precursor then coats the substrate or templateand then the coated particle such as the coated ceramic particle canthen be dried and subjected to heating to a temperature to form adensified material having desirable porosity and/or hardness. The typesof materials, characteristics, and parameters of the starting materialsand finished coated particles as described above apply equally here intheir entirety.

A solid or hollow alumina, aluminosilicate, or metal aluminate ceramicsphere can be obtained by coating a spherical template with an alumoxanesolution or metal doped alumoxane and then subsequent application ofheat to convert this sphere to alumina, aluminosilicate, or a metalaluminate. The alumoxane can comprise acetate-alumoxane. The sphericaltemplate preferably has a diameter in the size range of from about 25 to2000 microns. The solid or hollow spherical templates can be ceramic orcan be polystyrene or other polymeric materials. Even more preferably,the templates are cenospheres or synthetically produced microspheressuch as those produced from a blowing process or a drop tower process.In one embodiment, the solid or hollow spherical templates remain intact during the conversion process to alumina, aluminosilicate, or metalaluminate. The solid or hollow spherical templates pyrolyze, decompose,or can be otherwise removed during the conversion process to alumina,aluminosilicate, or metal aluminate. The wall thickness can be of anydesirable thickness. For instance, the wall thickness can be in a rangeof from about 25 to about 2000 microns. As an option, the surface of theformed alumina, aluminosilicate, or metal aluminate sphere can befunctionalized with a chemical moiety or chemical material, such as anorganic ligand, like a surfactant, and can provide surface wettingproperties which can assist in allowing additional ceramic precursor,which is the same or different from the earlier coating, to be applied.Then, additional heat conversion can occur to form the second ormultiple coating or shell on the already coated particle.

The solid or spherical templates can be first coated with a resin orpolymer and cured and then an alumoxane precursor or other similar typeof precursor can be subsequently coated onto the particle followed byheat conversion to form a sphere comprised of an outer alumina,aluminosilicate, or metal aluminate shell or similar type of metalcontaining coating. This resin coating or polymer coating can pyrolyze,decompose, or otherwise be removed during the conversion process. Thecoating used to coat the particles such as a solution of alumoxanenanoparticles can contain, for instance, from about 0.5 to about 20%alumoxane by weight of the coating solution. Other weights are possibleand permissible. The coating of the particles can occur such as bydipped coating, pan, Muller mixing, or fluid bed coating.

With respect to the polymers or resins that can be used to coat theparticles, these polymers include, but are not limited to, epoxies,polyurethanes, phenols, ureas, melamine formaldehyde, furans, syntheticrubber, natural rubber, polyester resins, and the like.

The proppants of the present invention while preferably used to propopen subterranean formation fractions, can be used in othertechnologies, such as an additive for cement or an additive forpolymers, or other materials that harden, or would benefit. Theproppants of the present invention can also be used as encapsulateddelivery systems for drugs, chemicals, and the like.

In another method of making the proppants of the present invention, acolloidal suspension containing polymeric beads can be suspended in anysolution such as an aqueous solution of a nanostructured coatingmaterial. The beads can then be covered with a nanostructured coatedmaterial to create a ceramic. The beads can then be dried andsubsequently heated to a first temperature which is sufficient toconvert the nanostructured coating material to a ceramic coating, suchas a densified coating. The temperature is preferably not sufficient todecompose the polymeric beads. Then, the polymeric beads can bedissolved, such as in a solvent, and extracted from the ceramic coating.Afterwards, the material can then be heated to a second temperature toform a hollow ceramic sphere of the desired porosity and/or strength.The nanostructure coating material can be as described above earlier,such as titania, alumina, chromium, molybdenum, yttrium, zirconium, orthe like, or any combination thereof. The nanostructure coating materialdispersed in the solution can be achieved using a sol-gel method,controlled flow cavitation, PVD-CVD, flame, plasma, high energy ballmilling, or mechanomade milling processes. The nanostructure coatingmedia can be a solution, such as alcohol, liquid hydrocarbon, orcombinations thereof.

In the present invention, the strength of the particle can be controlledby varying the wall thickness, the evenness of the wall thickness, thetype of nanoparticles used, or any combination thereof. Further, thesize of the particle can be controlled by varying the type, size, or anycombination thereof of the template used. The template can have a sizeof from about 1 nm to about 3000 microns.

In the present invention, in one or more embodiments, the templatematerial can be selected from wax, surfactant-derived liquid beads,seeds, shells, nuts, grain husks, grains, soils, powdered, ground, orcrushed agglomerates of wood products, powdered, ground, or crushedagglomerates of ceramic material, powdered, ground, crushed, or rolledorganics, silicas (glass beads), whey, cellulose, fly ash, animal eggs,rust, soap, bacteria, algae, and rubber.

More particular examples or seeds are rape seed, a poppy seed, a mustardseed, a kohl rabbi seed, a pea seed, a pepper seed, a pumpkin seed, anoil seed, a watermelon seed, an apple seed, a banana seed, an orangeseed, a tomato seed, a pear seed, a corn seed.

More particular examples of shells are walnut shell, peanut shell,pasticcio shell, or an acorn shell. More specific examples of grainhusks include, corn, maize, mailo, capher, or sorghum.

Another way to coat a particle in the present invention, can be with afluidized bed, spray drying, rolling, casting, thermolysis, and thelike.

Examples of powdered agglomerates of organic material include powderedmilk, animal waste, unprocessed polymeric resins, animal hair, plantmaterial, and the like. Examples of animal eggs include, but are notlimited to, fish, chicken, snake, lizard, bird eggs, and the like.Examples of cellulose templates include, but are not limited to algae,flower, plankton, ground cellulose such as saw dust, hay, or othergrasses, and the like. In general, the material coated can have a sizeof from about 100 to about 10,000 microns.

While the various embodiments of the present invention have beendescribed in considerable detail, the following provides additionaldetails regarding various embodiments of the present invention. It isnoted that the above disclosure of the proppants, methods of making, anduses applies equally to the following disclosure of various embodimentsof the present invention. Equally so, the following disclosure alsoapplies to the above embodiments of the present invention. Thesedisclosures are not exclusive of each other.

The proppant can comprise a template material and a shell on thetemplate material. The shell can comprise a ceramic material or oxidethereof or a metal oxide. The shell can contain one or more types ofceramic material, or oxides thereof, or metal oxides, or anycombinations thereof. The metal oxide can be a mixed metal oxide or acombination of metal oxides.

The template material can be porous, non-porous, or substantiallynon-porous. For purposes of the present invention, a substantiallynon-porous material is a material that is preferably at least 80 vol %non-porous in its entirety, more preferably, at least 90 vol %non-porous. The template material can be a hollow sphere or it can be aclosed foam network, and/or can be a non-composite material. Anon-composite material, for purposes of the present invention is amaterial that is not a collection of particles which are bound togetherby some binder or other adhesive mechanism. The template material of thepresent invention can be a single particle. In other words, the presentinvention can relate to a plurality of proppants, wherein each proppantcan consists of a single particle. In one or more embodiments of thepresent invention, the template material can be a cenosphere or asynthetic microsphere such as one produced from a blowing process or adrop tower process.

Though optional, the template material can have a crush strength of 5000psi or less, 3000 psi or less, or 1000 psi or less. In the alternative,the template material can have a high crush strength such as 3000 psi ormore, such as from about 5000 psi to 10,000 psi. For purposes of thepresent invention, crush strength is determined according to APIPractice 60 (2^(nd) Ed. December 1995). In one or more embodiments ofthe present invention, the template material having a low crush strengthcan be used to provide a means for a coating to be applied in order toform a shell wherein the shell can contribute a majority, if not a highmajority, of the crush strength of the overall proppant.

The template material can optionally have voids and these voids can bestrictly on the surface of the template material or strictly in theinterior of the template material or in both locations. As describeearlier, the shell can be sintered which can form a densified materialwhich preferably has high crush strength. For instance, the shell cancomprise sintered nanoparticles. These nanoparticles can be quite small,such as on the order of 0.1 nm up to 150 nm or higher with respect toprimary particle size. The nanoparticles can comprise primary particlesalone, agglomerates alone, or a combination of primary particles andagglomerates. For instance, the primary particles can have an averageparticle size of from about 1 nm to about 150 nm and the agglomeratescan have an average particle size of from about 10 nm to about 350 nm.The weight ratio of primary particles to agglomerates can be 1:9 to 9:1or any ratio in between. Other particle size ranges above and belowthese ranges can be used for purposes of the present invention. Theshell of the proppant can have an average grain size of about 10 micronsor less. The shell of the present invention can have an average grainsize of 1 micron or less. The shell of the proppant of the presentinvention can have an average grain size of from 0.1 micron to 0.5micron. In any of these embodiments, the maximum grain size can be 1micron. It is to be understood that maximum size refers to the highestgrain size existing with respect to measured grain sizes. With respectto any of these embodiments, as an option, at least 90% of all grainsizes can be within the range of 0.1 to 0.6 micron.

With respect to the shell, the shell can further comprise additionalcomponents used to contribute one or more properties to the shell orproppant. For instance, the shell can further comprise at least onesintering aid, glassy phase formation agent, grain growth inhibitor,ceramic strengthening agent, crystallization control agent, and/or phaseformation control agent, or any combination thereof. It is to beunderstood that more than one of any one of these components can bepresent and any combination can be present. For instance, two or moresintering aids can be present, and so on. There is no limit to thecombination of various agents or the number of different agents used.Generally, one or more of these additional agents or aids can includethe presence of yttrium, zirconium, iron, magnesium, aluminum, alumina,bismuth, lanthanum, silicon, calcium, cerium, one or more silicates, oneor more borates, or one or more oxides thereof, or any combinationthereof. These particular aids or agents are known to those skilled inthe art. For instance, a sintering aid will assist in permitting uniformand consistent sintering of the ceramic material or oxide. A glassyphase formation agent, such as a silicate, generally enhances sinteringby forming a viscous liquid phase upon heating in the sintering process.A grain growth inhibitor will assist in controlling the overall size ofthe grain. A ceramic strengthening agent will provide the ability tostrengthen the overall crush strength of the shell. A crystallizationcontrol agent will assist in achieving the desired crystalline phase ofthe shell upon heat treatment such as sintering or calcining. Forinstance, a crystallization control agent can assist in ensuring that adesirable phase is formed such as an alpha aluminum oxide. A phaseformation control agent is the same or similar to a crystallizationcontrol agent, but can also include assisting in achieving one or moreamorphous phases (in addition to crystalline phases), or combinationsthereof. The various aids and/or agents can be present in any amounteffective to achieve the purposes described above. For instance, the aidand/or agents can be present in an amount of from about 0.1% to about 5%by weight of the overall weight of the shell. The shell(s) can compriseone or more crystalline phases or one or more glassy phases orcombinations thereof.

The material or composition used to form the shell or coating caninclude reinforcing particulates. The particulates can be used forstrength enhancement or density control (reduce or increase density), orboth. The particulates can be included in the composition which formsthe shell and can be present in any amount such as from about 5 vol % to50 vol % or more, for example, from 5 vol % to 20 vol % of the overallshell. The reinforcing particulates can be ceramic material (e.g., oxideor non-oxide), metallic material (e.g., metal elements or alloys),organic material, or mineral-based material or any combination thereof.Ceramic particulates include, but are not limited to, alumina, zirconia,stabilized zirconia, mullite, zirconia toughened alumina, spinel,aluminosilicates (e.g., mullite, cordierite), silicon carbide, siliconnitride, titanium carbide, titanium nitride, aluminum oxide, siliconoxide, zirconium oxide, stabilized zirconium oxide, aluminum carbide,aluminum nitride, zirconium carbide, zirconium nitride, aluminumoxynitride, silicon aluminum oxynitride, silicon dioxide, aluminumtitanate, tungsten carbide, tungsten nitride, steatite, and the like, orany combination thereof. Metallic particulates include, but are notlimited to, iron, nickel, chromium, silicon, aluminum, copper, cobalt,beryllium, tungsten, molybdenum, titanium, magnesium, silver, as well asalloys of metals, and the like, or any combination thereof. Metallicparticulates may also include the family of intermetallic materials,such as the iron aluminides, nickel aluminides, titanium aluminides, andthe like. Organic particulates include, but are not limited to,carbon-based structures such as nanotubes, nanorods, nanowires,nanospheres, microspheres, whiskers of oxide, fullerenes, carbon fibers,nomex fibers, and the like, or combinations thereof. Mineral-basedparticulates include, but are not limited to, such materials as kyanite,mica, quartz, sapphire, corundum, including the range of aluminosilicateminerals that display high hardness and strength. Single crystalmaterials can be used. Resin material (e.g., organic resin(s)) with orwithout reinforcing particulates can be used as reinforcing material.

The morphology of the reinforcing particulates can have any aspectratio, such as aspect ratios ranging from 1 to 1,000 or greater. Thereinforcing particulates can have any shape or size. The reinforcingparticulates can be hollow, solid, or have voids. The diameter sizes canbe from about 10 nm to about 10 microns, such as from 1 to 2 microns.The same template modification to the surface can be done to thereinforcing surface. The particulates may be equiaxed, elongated grains,flakes, whiskers, wires, rods, nanowires, nanorods, platelets, spheresor any combination of them. The elastic and shear moduli of theparticulates may be similar, higher, or lower in value than that of theceramic matrix or other shell material. The thermal expansioncoefficient of the particulates may match that of the shell or may behigher or lower. The mean particle size of the particulates may rangefrom being equivalent to the particle size of the ceramic matrix (orother shell material) to significantly larger. Typical mean particlesizes of the reinforcement phase may range from 0.1 μm to 5 μm, with apreferential range from 0.2 μm to 1.0 μm. Other sizes can be used. Thepresence of particulates within the ceramic matrix (or other shellmaterial) generate beneficial residual strain fields, allow for cracktip blunting, provide a mechanism for crack trajectory deviation,provide a mechanism for crack tip capture, and/or provide a mechanismfor crack face bridging through the generation of ligands across cracks.

As an example, the incorporation of 20 vol % of 0.3 μm yttria stabilizedzirconia in a cordierite matrix ceramic shell has shown an improvementin the uniaxial crush strength by approximately 1200 PSI. Similarimprovements in the strength of the cordierite matrix through theincorporation of 20 vol % alumina particles with a mean particle size of0.2 μm has been observed.

The shell or coating can be obtained or made from (or include) a varietyof materials including waste or recycled materials. Examples of wastematerials include spent FCC catalysts, spent deNOx catalysts, spentautomotive catalysts, waste flyash and waste cenospheres derived fromcoal fired power stations and boilers, or ceramic-based catalystmaterials. These spent catalysts can be generally ceramic based systemsand can include such materials as alumina, mullite, and/or cordierite,with or without active phases present. The active phases may includeplatinum, rhodium, cerium, cerium oxide, titanium oxide, tungsten,palladium, and the like. The presence of the noble metals can have abeneficial effect as a reinforcing phase for the ceramic matrix. Anotheralternative coating material may be the use of silica flour (e.g., witha particle size of from 0.2 μM to 4 μm) that has been derived from thepowder milling of silica sand. In addition to silica sand, zircon sandmay also be used as a coating material for the template.

The template material can be a synthetic ceramic microsphere such as oneproduced from a blowing process or a drop tower process or can be acenosphere such as a hollow cenosphere. The template material can be afly ash particle or particles and can be a particle or particles derivedfrom fly ash. In more general terms, the template material can be ahollow spherical particle. The template material can be a precipitatorfly ash. The template material can be a blown hollow sphere. In otherwords, the hollow sphere can be naturally occurring or synthetic or canbe a combination.

The shell can be substantially non-porous. For instance, substantiallynon-porous means that at least 90% of the surface of the shell isnon-porous.

The shell can be substantially uniform in thickness around the entireouter surface of the template material. For instance, the thickness ofthe shell can be substantially uniform in thickness by not varying inthickness by more than 20% or more preferably by not varying more than10% in overall thickness around the entire circumference of the shell.The shell can be non-continuous or continuous. Continuous, for purposesof the present invention means that the shell entirely encapsulates orcovers the template material within the shell. Preferably, the shellfully encapsulates the template material.

With respect to the shell and the interaction of the shell and thetemplate material, the shell can essentially be a physical coating onthe template material and not react with the template material.Alternatively, the shell can react with one or more portions of thetemplate material such as by chemically bonding to the templatematerial. This chemical bonding may be ionic or covalent or both. As analternative, the shell or portion thereof can diffuse, infiltrate,and/or impregnate at least a portion of the template material. Asanother alternative, the shell or at least a portion thereof can adsorbor absorb onto the template material or a portion thereof.

With respect to the outer surface of the template material and theshell, the shell can be in direct contact with the outer surface of thetemplate material. Alternatively, one or more intermediate layers can bepresent in between the outer surface of the template material and theinner surface of the shell. The intermediate layer or layers can be ofany material, such as a polymer, resin, ceramic material, oxidematerial, or the like.

The proppants of the present application can, for instance, have aspecific gravity of from about 0.6 g/cc to about 4.0 g/cc. The specificgravity can be from about 1.0 g/cc to about 1.3 g/cc or can be fromabout 0.9 g/cc to about 1.5 g/cc, or can be from 1.0 g/cc to 2.5 g/cc,or from 1.0 g/cc to 2.4 g/cc, or from 1.0 g/cc to 2.3 g/cc, or from 1.0g/ee to 2.2 g/cc, or from 1.0 g/cc to 2.1 g/cc, or from 1.0 g/cc to 2.0g/cc, or from 1.0 to 3.5 g/cc, or from 2.0 to 3.0 g/cc, or from 2.5 g/ccto 3.5 g/cc. Other specific gravities above and below these ranges canbe obtained.

The proppant can have any of the crush strengths mentioned above, suchas 1000 psi or greater, 3000 psi to 10,000 psi, 10,000 psi to 15,000psi, as well as crush strengths above and below these ranges.

The shell of the proppant can have a wall thickness of any amount, suchas 5 microns to about 150 microns or about 15 microns to about 120microns. This wall thickness can be the combined wall thickness for twoor more shell coatings forming the shell or can be the wall thicknessfor one shell coating.

As stated, the proppant can be spherical, oblong, nearly spherical, orany other shapes. For instance, the proppant can be spherical and have aKrumbein sphericity of at least about 0.5, at least 0.6 or at least 0.7,at least 0.8, or at least 0.9, and/or a roundness of at least 0.4, atleast 0.5, at least 0.6, at least 0.7, at least 0.8, or at least 0.9.The term “spherical” can refer to roundness and sphericity on theKrumbein and Sloss Chart by visually grading 10 to 20 randomly selectedparticles. The template, such as the template sphere, can have aKrumbien sphericity of at least about 0.3, or at least 0.5 or at least0.6 or at least 0.8 or at least 0.9, and/or a roundness of at leastabout 0.1, at least about 0.3, at least about 0.5, at least about 0.7,at least about 0.8, or at least about 0.9.

The proppant can be a spray-coated shell. The shell(s) of the proppantscan be formulated by one coating or multiple coatings. More than oneshell can be present in layered constructions. The coatings can be thesame or different from each other.

The shell can comprise at least alumina, aluminosilicate, aluminate, orsilicate. For instance, the alumina, aluminate, or silicate can containcalcium, yttrium, magnesium, titanium, lanthanum, barium, and/orsilicon, or any combination thereof. One or more rare earth metals oroxides thereof can be present.

Another way to form the coating or shell around the template is by adirect particle deposition from a slurry, such as an aqueous-basedslurry, containing the precursor particles that form the shell orcoating. A sacrificial monolayer(s) can be first applied to the surfaceof the template and this monolayer can have the ability to inducecovalent bonding or other attachment mechanisms between the surface ofthe substrate and the particles in suspension that form the coating orshell. The treated template can then be immersed in an aqueous slurrycontaining the particles that form the coating or shell, thus resultingin particle attachment to the substrate surface through covalent bondingor other bonding mechanisms, wherein the covalent bonding can be betweenthe functional groups of the monolayer and the particles forming thecoating or shell. The coated template can then be subsequently driedand/or sintered to provide a desired shell. Generally, this technique ismore useful in forming thin shells and can be used to form anintermediate shell, wherein a subsequent strength-bearing shell can befurther created on top of this initial shell layer. By doing so, ahighly reactive interfacial layer can be achieved between the templatesurface and the strength-bearing shell, which can be a strength-bearingceramic shell. The material used to create the shell can be nano-sizedparticles, such as those described already herein. The sacrificialmonolayer can be organic material, such as organic material that allowscarboxy terminated self-assembled structures to be formed on the surfaceof the template. Another way to create a strength-bearing shell isthrough direct nucleation and deposition of the required species ontothe template surface, for instance, through a precipitation-typereaction. For example, the template particles can be suspended insolution in the presence of the material or precursor material used toform the shell. Precipitation of the material used to form the shell,such as ceramic material, can be accomplished through the adjustment ofthe pH of the solution, adjustment of temperature, and/or pressure or acombination of any of these. Through this process, the template surfacebehaves as a seed to initiate spontaneous precipitation and deposition.Precursors for precipitation include: titanium tetrachloride, titaniumoxychloride, titanium chloride, titanium oxysulphate, titanium sulphate,and titanium organo-metallic compounds such as titanium isopropoxide,calcium chloride, calcium hydroxide, aluminum chloride, aluminumnitrate, aluminum sulphate, and aluminum organo-metallics for examplealuminum isopropoxide.

For example, the formation of a yttrium oxide—yttria stabilized zirconiacomposite can occur by such a method. Yttrium oxide particles ofapproximately 2 μm mean particle size are suspended in a yttriumchloride—zirconium oxychloride solution with a concentration ofapproximately 100 g.L⁻¹ as dissolved oxide content. The pH of thesolution can be slowly raised in a controlled manner and the onset ofprecipitation can occur at a pH of approximately 5.5. The precipitatedparticles initiate on the surface of the yttrium oxide and attachthemselves covalently to the surface. Further particle precipitationoccurs and allows the formation of a shell of yttrium-zirconiumoxyhydroxide. Subsequent washing of the solid material followed by heattreatment yields a yttrium oxide core surrounded by a yttrium oxidestabilized zirconium oxide structure.

The coating or shell can be applied to the template or substrate using afluidized bed technique, as explained elsewhere in the presentapplication. The shell can be created or applied to the template orsubstrate using an interfacial reaction mechanism between the templatesurface and a solution incorporating the chemical species to achievethis interfacial reaction. The shell can be created or applied to thetemplate or substrate using a particle deposition technique, which caninvolve attracting and depositing particles from a slurry onto thesurface of the template to form a self-assembled structure. Theinterfacial reaction between the solution and the template surface canlead to the precipitation of precursor materials, such as ceramicprecursor materials, that decompose and densify, such as with theapplication of thermal energy or other energy sources. In creating theshell on the template or substrate, the surface charge of the templatecan be modified through the application of a functional species to thetemplate surface which can result in the electrostatic attraction ofparticles, such as ceramic particles in the suspension.

The template material can be from a naturally occurring material asdescribed earlier. The naturally occurring material can be seed, plantproducts, food products, and the like. Specific examples have beendescribed earlier.

The proppants of the present invention, for instance, can be made bycoating a template material with a formulation comprising a ceramicmaterial or oxide thereof or metal oxide to form a shell around thetemplate and then this formulation can be sintered to create thesintered shell having a densified structure. The shell preferably has amicrocrystalline structure. The sintering can occur at any temperatureto achieve densification of the ceramic material or oxide thereof ofmetal oxide such as from 700° C. to about 1,700° C. or about 800° C. toabout 1,700° C. Generally, sintering occurs by ramping up to thetemperature. The sintering temperature is the temperature in the oven orsintering device. As stated, the coating of the template material can beachieved by spray coating. For instance, in creating the shell, anon-alpha aluminum oxide can be coated onto a template material and thenupon sintering, form an alpha-aluminum oxide coating. The formulationcan be in the form of a slurry comprising the ceramic material or oxidethereof or metal oxide along with a carrier such as a liquid carrier.When spray coating, a spray coating chamber can be used such as a spraycoater from Vector Corporation, Model MLF.01. The formulation can beintroduced as an atomized spray and the template material is suspendedin air within the chamber during the coating of the template material.Ranges for key parameters for the spray coating process include: Airtemperature: 40°-90° C., Airflow: 90-150 liters per minute, Nozzle AirSetting: 10-25 psi. After coating, the sintering can occur.

With respect to the sintering, it is preferred that the sintering issufficient to densify the ceramic material or oxide thereof or metaloxide so as to form a continuous coating. The formulation can compriseat least one acid, surfactant, suspension aid, sintering aid, graingrowth inhibitor, glassy phase formation agent, ceramic strengtheningagent, crystallization control agent, and/or phase formation controlagent, or any combination thereof. One or more of these agents can bepresent. Again, as stated above, more than one type of the same agentcan be used such as more than one type of acid, more than one type ofsurfactant, more than one type of sintering agent, and so on. The amountof these agents can be any amount sufficient to accomplish the desiredpurposes such as from about 0.1% to about 5% by weight of the weight ofthe final shell.

As stated above, the present invention further relates to a proppantformulation comprising one or more proppants of the present inventionwith a carrier. The carrier can be a liquid or gas or both. The carriercan be water, brine, hydrocarbons, oil, crude oil, gel, foam, or anycombination thereof. The weight ratio of carrier to proppant can be from10,000:1 to 1:10,000 or any ratio in between, and preferably 0.001 lbproppant/gallon fluid to 10 lb proppant/gallon fluid.

In a more preferred example, the proppant can have the followingcharacteristics:

-   -   (a) an overall diameter of from about 90 microns to about 1,600        microns;    -   (b) spherical;    -   (c) and optional shell that is substantially non-porous;    -   (d) said proppant has a crush strength of about 1,000 or        greater, or 3,000 psi or greater;    -   (e) said optional coating has a wall thickness of from about 15        to about 200 microns, such as about 15 to about 120 microns;    -   (f) said proppant has a specific gravity of from about 0.6 to        about 4.0, such as about 0.9 to about 1.5 g/cc; and    -   (g) said template material is a hollow sphere.

Preferably, in this embodiment, the template material is a cenosphere oran aluminate, or a sintered aluminum oxide. The template materialpreferably has a crush strength of less than 3000 psi or less than 1000psi. The shell is preferably an alpha aluminum oxide coating.

For the proppants of the present invention, the shell can compriseMullite, Cordierite, or both. In one embodiment of the presentinvention, the formulation that is applied onto the template materialcan be prepared by peptizing Boehmite or other ceramic or oxidematerials with at least one acid (e.g., acetic acid) to form a sol-gelformulation comprising alumoxane. The formulation can be a slurrycomprising alumoxane along with a carrier such as a liquid carrier. Theslurry can contain one or more sintering aids, grain growth inhibitors,ceramic strengthening agents, glassy phase formation agents,crystallization control agents, and/or phase formation control agents,which can comprise yttrium, zirconium, iron, magnesium, alumina,bismuth, silicon, lanthanum, calcium, cerium, silicates, and/or borates,or oxides thereof, or any combination thereof.

The present invention can comprise a surface that comprises a ceramicmaterial or an oxide thereof or metal oxide wherein the surface (e.g.,polycrystalline surface) has an average grain size of 1 micron or less.The average grain size can be about 0.5 micron or less. The averagegrain size can be from about 0.1 micron to about 0.5 micron. The surfacehaving this desirable grain size can be part of a coating, shell, or canbe the core of a proppant or can be a proppant solid particle or hollowparticle. The surface can have a maximum grain size of 5 microns orless, such as 1 micron. Further, the surface can have grain sizes suchthat at least 90% of all grain sizes are within the range of from about0.1 to about 0.6 micron. The proppant can have a crush strength of 3000psi or greater or can have any of the crush strengths discussed above,such as 5000 psi or more or 10,000 psi or more, including from about5000 psi to about 15,000 psi. The ceramic material in this proppant canfurther contain yttrium, zirconium, iron, magnesium, alumina, bismuth,lanthanum, silicon, calcium, cerium, silicates, and/or borates, oroxides thereof or any combination thereof. The proppants can contain oneor more of sintering aid, glassy phase formation agent, grain growthinhibitor, ceramic strengthening agent, crystallization control agent,or phase formation control agent, or any combination thereof. Theproppant formulation can contain the proppant along with a carrier suchas a liquid carrier or a gas carrier.

With respect to this embodiment, a template material is optional. Theproppant can be completely solid, partially hollow, or completelyhollow, such as a hollow sphere. If a template material is present, anyone of the template materials identified above or below can be used.

Various methods exist to reduce the particle size distributions ofprecursor ceramic powders (or other powders that comprise the shell ortemplate). These methods include, but are not limited to, conventionalcomminution techniques. Examples include stirred media attrition mills,ball mills, hammer mills, impact mills, planetary balls mills, jetmills, micronising ring mills, or mechanochemical milling. Ball mills,hammer mills, jet mills and impact mills can yield a minimum meanparticle size of approximately 7 μm. Stirred media attrition mills andmicronising ring mills can yield a minimum mean particle size ofapproximately 0.2 μM. Planetary ball mills can yield a minimum meanparticle size of approximately 0.1 μm. Mechanochemical milling can yielda minimum mean particle size of approximately less than 0.05 μm (50nanometers) or less. Smaller particle sizes of the precursor ceramicpowders (or other powder) can lead to a reduction in sinteringtemperature. Thus, the mean particle size can be from 0.01 μm to 10 μmor from 0.05 μm to 7 μm or from 1 μm to 5 μm. With respect to lowersintering temperatures, it has been observed that grain growth isreduced which can lead to an improvement in the mechanical properties ofthe ceramic material (or other powder). Such properties include flexuralstrength, compressive strength, fracture toughness, and/or elasticmoduli (Young's, bulk and shear).

In all embodiments of the present invention, one or more proppants ofthe present invention can be used alone or in a formulation to prop opensubterranean formation fractions by introducing the proppant formulationinto the subterranean formation such as by pumping or other introductionmeans known to those skilled in the art. An example of a well completionoperation using a treating fluid containing proppants or particles isgravel packing. In gravel packing operations, particles referred to inthe art as gravel are carried to a subterranean producing zone in whicha gravel pack is to be placed by a hydrocarbon or water carrying fluid(or other carrier source, such as a fluid, energized fluid, foam, gas,and the like). That is, the particles are suspended in the carrier fluidwhich can be viscosified and the carrier fluid is pumped into thesubterranean producing zone in which a gravel pack is to be placed. Oncethe particles are placed in the zone, the treating fluid leaks off intothe subterranean zone and/or is returned to the surface. The gravel packproduced functions as a filter to separate formation solids fromproduced fluids while permitting the produced fluids to flow into andthrough the well bore. An example of a production stimulation treatmentutilizing a treating fluid having particles suspended therein ishydraulic fracturing. That is, a treating fluid, referred to in the artas a fracturing fluid, is pumped through a well bore into a subterraneanzone to be stimulated at a rate and pressure such that fractures areformed and extended into the subterranean zone. At least a portion ofthe fracturing fluid carries particles, referred to in the art asproppant particles into the formed fractures. The particles aredeposited in the fractures and the fracturing fluid leaks off into thesubterranean zone and/or is returned to the surface. The particlesfunction to prevent the formed fractures from closing whereby conductivechannels are formed through which produced fluids can flow to the wellbore.

While the term proppant has been used to identify the preferred use ofthe materials of the present invention, it is to be understood that thematerials of the present invention can be used in other applications,such as medical applications, filtration, polymeric applications,catalysts, rubber applications, filler applications, drug delivery,pharmaceutical applications, and the like.

The proppant of the present invention can be used as a compositematerial reinforcement phase, where the proppant serves to toughenand/or strengthen the composite structure and is distributedhomogenously within a matrix material. The matrix material can beceramic, polymeric, or metallic or a combination thereof.

The proppant can be used as a thermal insulating material either aloneor in combination with other materials as a cavity filling material oralternatively as a monolithic type structure, e.g. block, tube, sheet,rod, and the like. The proppant can be as an electrical insulatingmaterial, either as a cavity filling material or in combination withother materials as a monolithic type structure, e.g. block, sheet, tube,rod, and the like. The proppants can be used as an abrasive materialeither singly or incorporated into a resinous or polymeric matrix andformed into discs, rods, sheets, cups, wheels, and the like. Theproppants can be used as substrates for catalysts. The proppants can beused as column packings for chromatography applications. The proppantscan be used as reflux tower packings in distillation columns. Thepresent invention includes a matrix comprising a plurality of theproppant of the present invention and at least one matrix material. Theproppant can have the outer surface of the proppant treated afterforming to improve the hydrophobic nature of the proppant. The proppantcan have an outer surface that is treated after forming to improve thehydrophilic nature of the proppant. The proppant can have an outersurface that is treated after forming to produce a hydro neutralsurface.

U.S. Pat. Nos. 4,547,468; 6,632,527 B1; 4,493,875; 5,212,143; 4,777,154;4,637,990; 4,671,909; 5,397,759; 5,225,123; 4,743,545; 4,415,512;4,303,432; 4,303,433; 4,303,431; 4,303,730; and 4,303,736 relating tothe use of proppants, conventional components, formulations, and thelike can be used with the proppants of the present invention, and areincorporated in their entirety by reference herein. The processesdescribed in AMERICAN CERAMIC SOCIETY BULLETIN, Vol. 85, No. 1, January2006, and U.S. Pat. Nos. 6,528,446; 4,725,390; 6,197,073; 5,472,648;5,420,086; and 5,183,493, and U.S. Patent Application Publication No.2004/0012105 can be used herein and is incorporated in its entiretyherein. The proppant can be a synthetic proppant, like a syntheticcenosphere template, with any shell.

Sol-gel routes to forming monoliths or films, such as thicker than about1 micron, can suffer from severe cracking or warping during the dryingor gel consolidation process. Low solids-weight loadings result in largevolumetric shrinkage during the drying process. Furthermore, crackingcan be the result of relief of differential capillary stresses acrossthe dimensions of the gel or film as drying and shrinkage occur. Totalcapillary stress present in the film can be a function of particle size,and decreases as particle size increases. As a result, films ormonoliths formed from larger particle sizes can have a decreasedtendency to incur cracking stresses during the shrinkage and dryingprocess.

In a peptized formulation, such as a boehmite gel formulation, ablending of boehmite particles of varying dispersed size (e.g., 90%large, aggregated crystallites and 10% small, single crystallites)results in a lower number density of pores, as well as larger size ofpore in the corresponding dried gel, thereby reducing drying stress.Thus, tailoring the particle size and blend of primary particles in thesol-gel formulation can confer control over crack formation for a givendrying process. The particles have varying dispersed sizes in thesol-gel stage, but examination of the microstructure of the dried gelfragments reveals that only crystallites are distinct from one anotherin the green packing. This shows that the small particles uniformly fillthe interstices of the larger particles, resulting in a well-structuredgreen film.

The proppants can be unagglomerated and each proppant can have a singleparticulate template with a shell formed from one or more layers. Thus,in one or more embodiments, there can be a 1:1 ratio with respect toshell to template, meaning there is one shell for each template particleor sphere. Thus, the present invention relates to a plurality ofproppants which comprise individual template particles or spheres thatare coated individually to form individual templates having a shellaround each template sphere or particle. Further, in one or moreembodiments, the population of proppants is consistent prior tosintering and/or after sintering. In other words, substantially eachproppant (e.g., over 90% or over 95% or from 95% to 99.9%) of theproppant population in a plurality of proppants have a continuouscoating around the template to form the shell and/or the shell has auniform thickness around the template (e.g., the shell thickness doesnot vary more than +/−20% in thickness, such as +/−10% or +/−5% inthickness around the entire template) and/or each proppant in theplurality of proppants is substantially flaw-free.

The template can have one or more voids in the template wherein the oneor more voids amount to at least 20% void volume (or at least 30% voidvolume) in the template wherein the percent is based on the entirevolume of the template. The void volume can be from 25% to 95%, or from50% to 95%, or from 60% to 95% or more, or from 70% to 90%, or from 75%to 90%) or from 80% to 90%.

There can be a center void located in the middle of the template,especially when the template is a sphere. The template can have a centervoid, but also other voids located throughout the template.

The shell of the present invention can have the ability to significantlyadd strength to the overall proppant even when the template (by itself)has a low point crush strength, such as a crush strength of 50 psi to1,000 psi, or from 100 psi to 500 psi, or from 100 psi to 200 psi. Theshell of the present invention can raise the crush strength of theoverall proppant by 50%, 100%, 200%, 300%, 400%, 500%, or more. Thepresent invention has the ability to take surface imperfections existingin the template, which could cause a failure in the proppant, andminimize or eliminate these defects in the template surface by forming ashell around the template. Thus, the shell of the present invention notonly adds strength, such as crush strength to the overall proppant, theshell of the present invention has the ability to minimize surfacedefects in a template, such as a template sphere.

The template can be a material that can withstand a sinteringtemperature of at least 700° C. for 10 minutes in air or an oxidizingatmosphere. Preferably, the material forming the template can withstandsintering at a temperature range from 800° C. to 1,700° C. However,templates which can withstand a sintering temperature (in air or anoxidizing atmosphere) of 800° C., or 900° C., or 1,000° C., or 1,200°C., or 1,400° C. for 10 minutes or other temperatures can be used.

Any template material can be formed into a suitable template sphere forpurposes of the present invention by reducing the size of the startingmaterial, such as perlite, vermiculite, pumice, or volcanic materials bygrinding, such as in an attrititor mixer and the like. The grinding willreduce the size to the desirable size of the template as mentionedherein. Alternatively, the template sphere can be formed by combiningsmaller particles to form a composite particle.

The proppant can have a substrate or template that comprises aninorganic material, a metal oxide, or a combination of metal oxidesand/or inorganic materials. Examples include, but are not limited to,oxides of aluminum, silicon, titanium, magnesium, sodium, potassium, andthe like. Alloys of these various metal oxides can also be used or beadditionally present.

The substrate or template (or any layer of the shell) can be a mineralor contain a mineral, such as containing a mineral phase. The templateor substrate, in one or more embodiments, can be or contain perlite,vermiculite, pumice, or volcanic materials that optionally areexpandable, such as expandable with the application of heat.

The template or substrate (or any layer of the shell) can be a mixtureof two or more metal oxides in any proportions. For example, thetemplate or substrate can be a mixture of aluminum oxide and siliconoxide. The template or substrate can be formed into a solid sphere or ahollow sphere or a sphere having one void or multiple voids by a varietyof methods. The proportions of the metal oxides, when present as amixture, can be 1:99 to 99:1% by weight. For instance, the proportionscan be 85:15% to 70:30% or 60:40% to 27:73% by weight percent. A fluxingagent(s), such as sodium oxide, calcium oxide, magnesium oxide, lithiumoxide, and/or potassium oxide can be used as part of the templateformation, such as in amounts up to 10 wt %.

In addition to, as an alternative, or in any combination with otherembodiments described herein, the template, the shell (or one or morelayers comprising the shell), or any reinforcing material can be surfacemodified, such as with the addition of silicon oxide, sodium oxide,potassium oxide, calcium oxide, zirconium oxide, aluminum oxide, lithiumoxide, iron oxide, cordierite, spinel, spodumene, steatite, a silicate,an alumino silicate material, an aluminum containing material, a siliconcontaining material, an aluminum-silicon containing material, asubstituted alumino silicate clay or any combination thereof.

The template or substrate can be formed into a hollow sphere or a spherehaving one or multiple voids by any method, such as by a coaxial nozzlemethod, with the use of a blowing agent(s), by the solidification of thesphere from a melt phase, such as achieved with cenospheres derived fromcoal fly ash, or the aggregation of a plurality of smaller hollowspheres to form a substantially spherical agglomerate preferably withlow specific gravity.

The template or substrate can have one void, multiple voids, a porousnetwork of unconnected or interconnected pores or voids. The voids orpores can have substantially the same size or differing sizes. One ormore of the voids or pores can be interconnected and/or the voids orpores can be closed voids or pores, meaning not interconnected withother voids or pores. The size of the voids or pores can be from about0.05 μm to about 500 μM, such as from about 1 μm to about 300 μm. In oneor more embodiments, the template or substrate can be highly porous,wherein 60 vol % or more of the overall volume of the template orsubstrate is porous through voids, open voids or pores, closed voids orpores, or any combination thereof. The template or substrate can beporous, such that the voids and/or pores amount to 10% by volume to 95%by volume of the overall volume of the template or substrate, othervolume percents include, but are not limited to, from about 30% to 95%by volume, from about 35% to about 80% by volume, from about 50% toabout 75% by volume, wherein, as stated above, these percents are basedupon the percent of the entire volume of the template or substrate.

The substrate or template can be any naturally occurring material orsynthetic material. More particularly, and simply as an example, thesynthetic material can be any synthetic material that can be formed in asphere or substantially spherical shape. For instance, syntheticcenospheres can be used as the template or substrate of the presentinvention.

For purposes of the present invention, the template or substrate canhave one or more coatings located on a core material (composite ornon-composite material). The template or substrate can be a singleparticle or be made up of multiple particles formed, for instance, intoa composite single particle. The present invention has the ability totake various types of substrates or templates and form them intosuitable proppants that can have the proper crush strength, buoyancy,and the like. One way of transforming templates or substrates is withthe application of one or more shells on the template or substrate, suchas ceramic coatings, as described herein.

The shell in one or multiple layers can be a metal oxide or acombination of metal oxides such as oxides of aluminum, silicon,zirconium, magnesium, or any combination thereof. The shell can providea substantially higher crush strength to the overall proppant, such asexceeding 1,500 psi, such as 2,500 psi or higher or at least 5,000 psi(e.g., from above 5,000 psi to 15,000 psi). One way of achieving thisincreased crush strength is incorporating a secondary phase, one or moredopants, and/or the use of multiple layers to form the shell around thetemplate or substrate. In one or more embodiments, the shell can containreinforcing material, such as particulates, fibers, whiskers, orcombinations thereof. The reinforcing material can be present in theshell in an amount from about 1 wt % to about 25 wt %, and moreparticularly from about 5 wt % to about 15 wt %, based on the weightpercent of the shell. The particulates or fibers typically can have asize of from about 0.1 μm to about 5 μm, more particularly from about 1μm to about 3 μm. The reinforcing material can be uniformly distributedthroughout the surface area of the shell. Examples of particularreinforcing materials include, but are not limited to, carbon black,fiberglass, carbon fibers, ceramic whiskers, ceramic particulates,and/or metallic particles. The shell can contain secondary phases, suchas, but are not limited to, inorganic or ceramic phases. Examplesinclude metal oxide(s), metal carbide(s), metal nitride(s), or anycombination thereof. Zirconium oxide, zirconium carbide, and zirconiumnitride are examples. The zirconium oxides (zirconium carbides, and/orzirconium nitrides) can be stabilized in a useful crystallographicphase, such as through the use of one or more elements, such as metals.For instance, zirconium oxide, such as a tetragonal phase of zirconiumoxide, can be stabilized through the additions of the oxides ofmagnesium, calcium, cerium, yttrium, scandium, or any combinationthereof. The carbides or nitrides of zirconium can be stabilized throughthe use of silicon, titanium, tungsten, aluminum, boron, or anycombination thereof. The stabilizers can be present in any amount, suchas from about 3.5 wt % to about 5.5 wt % based on the weight of metaloxide, such as zirconium oxide. Other examples of amounts include fromabout 10 wt % to about 17 wt %, based on the weight of metal oxide, suchas zirconium oxide. The formation of the metal carbides, such ascarbides of silicon, titanium, zirconium, tungsten, aluminum or boron(optionally with the one or more stabilizers), can be formed underelevated temperatures (such as temperatures from about 500° C. to about1,200° C.) in an atmosphere of carbon monoxide, where a source(s) ofsilicon, titanium, zirconium, tungsten, aluminum, or boron (or otherelemental particles) can come in contact with carbon to form the carbidephase, for instance, zirconium carbide. An unsintered shell can becoated with a carbon containing material (such as carbon black, pitch,charcoal, or coke derived from coal) prior to heat treating to form acarbide. In general, the shell can comprise a metal oxide, a metalcarbide, a metal nitride, or any combination thereof, derived from asilicon source, titanium source, tungsten source, zirconium source,aluminum source, boron source, or any combination thereof.

Similarly, the metal nitrides (e.g., zirconium nitride) (having one ormore stabilizers) can be formed under elevated temperatures in anatmosphere containing ammonia, nitrogen, nitrous oxide, or anycombination thereof. Examples of elevated temperatures include, but arenot limited to, from about 500° C. to about 1,800° C.

The shell can contain one or more dopants, such as materials to improvethe densification of the shell material, retard the densification of theshell material, and/or to improve the susceptibility of the shellmaterial to external influences during the sintering process.

The shell can be surface modified, for instance, by the addition of oneor more inorganic materials or phases, or the attachment (e.g., chemicalattachment or bonding) of one or more chemical groups, such ashydrophilic groups or hydrophobic groups. The chemical groups can besurfactants, polymers, ionic groups, ionizable groups, acid groups,salts, surface active agents, and the like. The surface modification canimprove the surface morphology of the proppant, especially after theproppant is a sintered proppant. The inorganic material or phases usedfor surface modification can include glassy materials, such as siliconoxide, alone or with the addition of oxides of sodium, potassium,calcium, zirconium, aluminum, lithium, iron, or any combination thereof.The amount of the silicon oxide can be from about 70 wt % to about 99 wt%, such as from about 85 wt % to about 95 wt %, and the addition of theone or more other oxides, such as sodium oxide and the like, can be fromabout 1 wt % to about 15 wt %, such as from about 2 wt % to about 10 wt%. The surface modification of the shell can include the application ofone or more organic materials (e.g., aliphatic compounds, ioniccompounds, surfactants, aromatic compounds, polymeric compounds) or theapplication of an organic phase(s). The organic material or chemicalgroups can be bonded to the shell surface or adsorbed, or absorbed orotherwise attached. The organic material or organic phase can modify theproppant's propensity to interact with aqueous solutions, thus makingthe proppant either hydrophobic, hydrophilic, or hydro-neutral. Thesurface modification of the shell can include the use of substances thatare effectively activated by temperature elevation of the proppant toyield a modification of the proppant transport fluid (e.g., breaking thegel used to transport the proppant through the subterranean regions).The surface treating, which can occur after sintering of the proppant,can have the ability to improve one or more chemical and/or mechanicalproperties, such as enhanced transportability.

The proppants of the present invention can be designed to improvereservoir flow rates by changing the hydrophilic properties of theproppants themselves. The hydrophilic nature of current proppants causeswater to be trapped in the pore spaces between proppants. If this watercould be removed, flow rates would be increased. A route to theprevention of this entrapped water within the proppant pack, is tofunctionalize the surface of the proppant particles with at least onechemical substituent such that more hydro-neutral or hydrophobic surfacewetting properties of the particles are achieved. Such chemicalsubstituents include, but are not limited to, functionalizedcarboxylate, phosphate, or esters, where the functionalized group can bean alkyl group, or other moiety conferring a degree of surfacehydrophobicity or hydrophilicity. In particular, the surface can befunctionalized with hydrophobic carboxylic acids such as hexanoic acidor parahydroxybenzoic acid, or methoxy, methoxy(ethoxy), ormethoxy(ethoxyethoxy)acetic acids. Furthermore, the carboxylatealumoxanes of each of these acids may also be used for the surfacefunctionalization. The chemical groups mentioned throughout can be usedas well for functionalization. These functionalizations of the proppantsurface allow for varying of the wetting properties continuously withinthe range and spectrum of hydrophilicity, hydro-neutrality, andhydrophobicity of the surface.

The shell or one or more layers comprising the shell can be densified bya variety of methods. Examples of methods that can be used to densifythe shell or a layer of the shell include, but are not limited to,liquid phase sintering, reactive phase sintering, and/or solid statesintering. As a more specific example, the densification can be achievedby indirect radiant heating, direct infrared radiation, directconduction of the heat flux from the environment to the proppant shell,excitation of the constituent molecules of the shell, and consequentheating of the shell by electromagnetic radiation, inductive coupling ofthe shell material to an external excitation field of alternatingcurrent for instance, with a frequency from 5 to 1,000 HZ. The heatingof the shell by electromagnetic radiation can be at a frequency from 2to 60 GHZ, such as that generated by a magnetron. The pressure assistedsintering can be carried out with an application of external gaspressure to the system during heat treatment, with pressures, forinstance, ranging from ambient to 1,500 PSIG.

For purposes of liquid phase sintering, additives may be used, such asmetal oxides. Examples include, but are not limited to, oxides ofaluminum, silicon, magnesium, titanium, lithium, sodium, calcium,potassium, or any combination thereof. The amount of the additives canbe from about 0.1 wt % to about 5 wt %, such as from about 0.25 wt % toabout 2 wt %, per weight of oxide in the shell. The liquid phasesintering additives can be introduced by way of chemical mixing,coating, or dissociation of a secondary phase in the coating.

With respect to the secondary phases which may be present with theshell, the secondary phases can be achieved by forming the secondaryphase in-situ with the other ingredients used to form the coating, whichultimately forms the shell on the template or substrate. The secondaryphase can be prepared separately and then added to the composition usedto form the shell by blending or other introduction techniques.

The shell can comprise multiple layers, such as two or more layers, suchas two, three, four, or five layers. The layers can be the same ordifferent from each other. Various layers can be used to achievedifferent purposes or physical or chemical properties. For instance, onelayer can be used to improve crush strength, and another layer can beused to achieve buoyancy. For example the use of an aluminosilicatematerial as a layer material would produce an interfacial layer betweenthe template and subsequent layers to improve boding of the layers. Theuse of an aluminosilicate layer, by virtue of its lower elastic modulus,would also improve the mechanical properties of the system due to theability to inhibit crack propagation. Examples of such a materialinclude, mullite, cordierite, and doped aluminosilicates with a dopantphase such as, but not limited to, lithium, magnesium, sodium. The lowerspecific gravity of the layer can reduce the overall specific gravity ofthe proppant, thus increasing the buoyancy of the proppant. The loadbearing layer can be any of the widely available engineering ceramicssuch as, but not limited to aluminum oxide, stabilized zirconium oxide,carbides and nitrides. By using different layers, one or moresynergistic results can be achieved with respect to the performance ofthe proppant. For instance, the use of two or more layers, which may bethe same or different, can achieve synergistic improvement with respectto the generation of useful strain fields in the system, beneficialcrack tip influences, or a combination of these properties. The thermalexpansion coefficient mismatch between layers may generate residualcompressive strain fields in one or more of the layers leading toapparent increases in fracture toughness and strength of the overallproppant system. The presence of an interface between layers will leadto deviation and/or trapping of an advancing cracktip during loading toimprove the apparent fracture toughness and strength of the overallproppant system.

Besides the various techniques already mentioned, the shell can beformed on the template or substrate by a fluidized bed method. Forinstance, the shell can be formed through the use of heated fluidizingair, dried fluidizing air at ambient temperature, or dried heatfluidizing air. The ambient temperatures would be from 15 to 30° C. andthe heated fluidizing air would be from about 50 to about 200° C. Thedew point of the fluidizing air can be below 0° C. The temperature ofthe fluidizing air can be from about 25° C. to about 200° C. The volumeof the fluidizing air admitted into the chamber can be the same orvaried to effect adequate fluidization of the templates during thecoating process. For example, as the coating builds, the mass of theindividual particles increases such that they may no longer remainadequately fluidized, thus an increase in air velocity (volume) can beused to maintain the particles in suspension. The material forming theshell can be applied to the templates in the fluidizing bed through theuse of a spray nozzle. The spray nozzle can have both single and/or dualfluid designs. The single fluid design can effect the atomization of thesolution that forms the shell through, for instance, the effects ofpressure reduction at the orifice of the nozzle. The dual fluid nozzlecan effect the atomization of the solution that forms the shell throughthe addition of atomizing air to the solution either prior to exitingthe nozzle or after exiting the nozzle, i.e., internal mix or externalmix design.

The template or substrate (and/or shell) can be modified, such assurface modified, through a variety of techniques. For instance, thesurface of the template or substrate can be modified through one or moreheat treatments prior to the formation of the shell on the template.Another form of surface treatment can be to modify chemically thesurface of the template, such as by glazing, application of a bond coat,or chemical etching. The chemical modification can improve theperformance and/or stabilize the template surface. Removal of residualimpurities, cleaning of the surface, reduction of residual staindistributions prior to coating, increasing the microscopic roughness ofthe surface to improve coating bond strength or removing and/orimproving the morphology of protuberances. Furthermore, the modificationcan be achieved by the preferential removal of one or more constituentphases on the template. For example, a wash with caustic soda maypreferentially dissolve silica from the template.

Examples of templates or substrates that can be used in the presentinvention include metal-fly ash composites, which can contain metaland/or metal alloys. Examples include those set forth in U.S. Pat. No.5,899,256, incorporated in its entirety by reference herein. The metalor metal alloy can be aluminum or an aluminum alloy. Further, themetal-fly ash composites contain cenosphere fly ash particles or otherfly ash particles. The template or substrate used in the presentinvention can be particles, such as spheroidal particles of slag and/orash. These particles, for instance, can contain SiO₂, Al₂O₃, and/or CaO.Other oxides can be present, such as other metal oxides. Examples ofparticles can include those set forth in U.S. Pat. No. 6,746,636, whichis incorporated in its entirety by reference herein.

The template or substrate can be a sintered composite particulatecontaining nanoparticles and/or a clay material, bauxite, alumina,silica, or mixtures thereof. The nanoparticles can be hollowmicrospheres, such as hollow mineral glass spheres, such as SPHERELITES,CENOLIGHTS, SCOTCH LIGHT, or Z-LIGHT SPHERES. The template or substratecan be formed of clay or hydrided aluminum silicate, bauxite, containing30 to 75% Al₂O₃, 9 to 31% H₂O, 3 to 25% FeO₃, 2 to 9% SiO₂, and 1 to 3%TiO₂.

The template or substrate (or the shell) can have a resin or polymercoating on it to aid in dispersibility, to aid in receiving a shell ofthe present invention, or for processing reasons. The template orsubstrate (or the shell) can contain nanoparticles, such as nanoclays,carbon nanofibers, silica, carbon nanotubes, nanoparticle minerals, suchas silica, alumina, mica, graphite, carbon black, fumed carbon, fly ash,glass nanospheres, ceramic nanospheres, or any combination thereof. Thenanoparticles can have a size of 10 nanometers to 500 nanometers. Thetemplate or substrate can be natural sand, quartz sand, particulategarnet, glass, nylon pellets, carbon composites, natural or syntheticpolymers, porous silica, alumina spheroids, resin beads, and the like.It is to be understood that the term “template or substrate,” as usedthroughout the application, includes template spheres, and preferablyhaving at least one void.

The template or substrate (or the shell) can be a porous particulatematerial, which can be treated with a non-porous coating or glazingmaterial. For instance, the porous particulate material can be porousceramic particles, such as those set forth in U.S. Pat. No. 5,188,175,incorporated in its entirety by reference herein. The non-porous coatingor glazing material can be a resin or plastic.

The template or substrate (or the shell can contain) can be syntheticmicrospheres, such as ones having a low alkali metal oxide content.Examples include those set forth in U.S. Patent Application PublicationNo. 2004/0079260, incorporated in its entirety by reference herein. Thetemplate or substrate can be spherical material made from kaolin clayhaving an aluminum content distributed throughout the pellet, such asthose described in U.S. Patent Application Publication No. 2004/0069490.

The template or substrate can be sintered, spherical composite orgranulated pellets or particles, such as ones containing one or moreclays as a major component and bauxite, aluminum, or mixtures thereof.The pellets can have an alumina-silica ratio from about 9:1 to about 1:1by weight.

The template or substrate (or the shell) can be porous ceramic particleshaving an average particle diameter of 50-2,000 microns. The porousceramic particles can have pores averaging from about 1 to 100 micronsin diameter, and other pores averaging from about 0.001 to 1 micron indiameter. The ceramic material can include alumina, silica, zirconia,titania, zirconium aluminum titanate, or nitrides thereof, carbidesthereof, or mixtures thereof. The particles can have an average surfacearea (BET) of about 1 to 50 m²/g. The particles can have an averagesurface area (BET) of from about 100 to 500 m²/g.

The shell can comprise one or more layers. When multiple layers comprisethe shell, the layers can be the same or different from each other. Thethickness of each layer can be the same or different. As stated, one ormore layers can comprise an inorganic or ceramic material, such asalumina or other ceramic or inorganic materials as described herein. Oneor more of the layers can be a toughened layer, such as a toughenedinorganic or ceramic layer, such as alumina. For instance, the ceramicor inorganic layer that comprises at least one layer of the shell, canbe strengthened or toughened by the addition or presence of metal oxide,metal nitrides, and/or metal carbides, and the like (e.g., zirconiumoxide, zirconium nitride, and/or zirconium carbide). The strengthenedlayer can be achieved by either adding an additional layer comprising aninorganic or ceramic material, such as metal oxide, metal nitrides, ormetal carbides, or in the alternative or in addition, thepreviously-applied layer containing the inorganic or ceramic materialcan be converted or chemically altered to contain a nitride, a carbide,or the like. Or, a nitride, carbide, or both can be added. Either optionis possible. The nitrides can contain or be a nitride of Si, Al, Ti, Zr,W, Bo, and the like. The carbides can contain or be a carbide of Si, Zr,W, Al, Ti, Bo, and the like.

The shell can comprise one or more layers, and can provide a surfacerepairing of the template surface, such as a template sphere surface.The surface repairing can comprise infiltrating cracks or flaws on ashell and/or template with a suspension comprising aluminum silicate,ceramic particles (nano-sized) alumoxane, mullite, or other minerals) ora combination thereof.

The surface repair can be achieved, for instance, by providing a glazinglayer on the template surface. The glazing layer can at least partiallyinfiltrate or penetrate below the top or outer surface of the templatesurface, such as in cracks or flaws in the template sphere. Forinstance, a thin layer (e.g., 0.5 micron to 10 microns) of silica,mullite, cordierite, spodumene, or other inorganic, mineral-containing,or ceramic-containing materials. (This materials as well all materialsmention throughout can be used as the shell material and/or templatematerial). This glazing layer can form part of the overall shell andserve as one layer of the shell. Preferably, this surface repairing ofthe template is achieved by glazing the immediate surface of thetemplate. Another form of repairing the surface of the template prior toformation of the shell on the template surface can be achieved by heattreatment, which can densify, consolidate, or otherwise repair cracks orother flaws in the surface of the template. The heat treatment can occurat any temperature depending upon the composition of the templatematerial, for instance, at temperatures of from about 500° C. to 1,700°C. for a time sufficient to surface repair (e.g., 10 minutes or more).In addition, as another option to surface repairing of the templatesurface, the cracks or flaws can be infiltrated with suspensions ofalumoxanes or other inorganic (e.g., mullite, metal oxides) orceramic-containing materials, such as alkyl-alumoxanes,methyl-alumoxanes, and the like. Further, dopants containing one or moreof the alumoxanes or other materials can be used. Also,alumina-containing slurries and mullite-containing slurries, as well asinorganic or ceramic-containing slurries, can be used. Preferably, theparticle size of the alumoxane, alumina, mullite, or other inorganic orceramic-containing particles are small enough to fit within the crackedsurface, such as particles on the nano-scale level, for instance, from 1nanometer to 1,000 nanometers.

One of the layers comprising the shell can be a resin orpolymer-containing layer. For instance, the resin or polymer-containinglayer can be the outermost layer of the shell and can optionally have atacky surface, which permits the proppant to have the ability to remainin the subterranean formation during hydrocarbon recovery. Also, shouldthe proppant have a structure failure, an outer resin coating, such asone that is tacky, can permit the failure of the proppant to stay in theproppant location in the subterranean formation without interfering withhydrocarbon recovery. The resin coating or polymer layer can be anythickness as described herein with respect to any other layer of theshell, such as from about 5 microns to 150 microns. The resin or polymerlayer can be located anywhere as part of the shell, such as theinnermost layer of the shell, the outermost layer of the shell, or oneof the intermediate layers of the shell depending upon the purpose ofthe resin or polymer layer.

The template material or sphere can be or contain a geopolymer orcontain a pore forming or pore containing material (e.g., dissolvable ordecomposable material) which can be subsequently subjected to chemicaletching or a burn-out, wherein portions of the geopolymer or otherpore-forming material can be removed.

The template, such as the template sphere, can comprise ceramic materialor inorganic material, which can be present with pore or void-formingmaterial, wherein the pore or void-forming material is removable by anyprocess, such as chemical etching, heating, and the like. Once thepore-forming or void-forming material is partially or completelyremoved, the template sphere or material is achieved, which has thedesirable void volume percent as described herein.

The shell can be formed on the template, such as the template sphere,while the template or template sphere is in a green state, and then theentire proppant having the template and the shell can then be subjectedto sintering or heat treatment to form or consolidate the shell andconsolidate or calcine the template sphere as well.

The template material, such as the template sphere, can have a specificgravity of from 0.25 to 0.85 g/cc. This specific gravity for thetemplate sphere can be especially useful when coating the templatesphere to form the shell, especially when the coating is being achievedwith the use of a fluidized bed.

The template sphere or material is preferably a particle not formed bygranulation or agglomeration techniques, but is a single individualcontinuous particle having one or more voids.

The template or substrate can be a hollow microsphere made by thecoaxial process described in U.S. Pat. No. 5,225,123, incorporated inits entirety by reference herein. This process includes the use offeeding a dispersed composition in a blowing gas to a coaxial blowingnozzle, wherein the coaxial blowing nozzle has an intercoaxial nozzlefor blowing gas and an outer nozzle for the dispersed particlecomposition. The hollow microspheres can be made from ceramic materials,glass materials, metal materials, metal glass materials, plasticmaterials, macroparticles, and the like.

The proppants of the present invention can be used in a variety ofareas. The proppants can be used as substrates as semi-permeablemembranes in processes for carrying out gas and liquid separations andfor use as substrates for catalysts and enzymes. The proppants can beused in processes for the manufacture and purification of pharmaceuticalor chemical products, for instance, using or derived fromgenetically-engineered bacteria, natural living organisms, and enzymes.The proppants of the present invention can be used as containers forliquids, adsorbents, absorbents, or catalysts or as containers forchemical agents whose release is subject to predetermined control (e.g.,controlled slow release).

The proppants of the present invention can be used in one or more of thefollowing areas as a composition, an additive, and/or to fully replaceor partially replace the filler or reinforcing agent conventionallyused, using similar or the same amounts, or lesser amounts, to achievethe same or improved properties: proppants for oil and gas industry,lightweight high strength fillers for polymers, as a component in amatrix system, syntactic foams for aerospace applications, highperformance fillers for cement and concrete, high performance refractorymaterials, high strength, lightweight insulating materials, carriers forcatalysis systems, water treatment systems, high strength, lightweightparticulate reinforcements for polymer matrix composites, high strength,lightweight particulate reinforcements for ceramic matrix composites,high strength, lightweight particulate reinforcements for metal matrixcomposites, high performance casting sand for metal castingapplications, or friction reducing fillers for polymer processingsystems (e.g. extrusion, die casting, etc). Matrix materials mayinclude, but are not limited to the following: polymeric systems such aspolyesters, epoxies and urethanes, polyethylenes, polypropylenes, andthe like, calcium silicate based cement systems, calcium aluminate basedcement systems, foamed polymeric systems, extruded polymeric systems,and ceramic systems.

The proppant of the present invention can be prepared by forming atemplate sphere and providing a shell around the entire outer surface ofthe template sphere, and then sintering the shell to form a continuoussintered shell. The sintering can be liquid phase sintering, reactivephase sintering, or solid state sintering, or any combination thereof.The sintering can comprise indirect radiant heating, direct infraredradiation, direct conduction of heat flux from an environment to theproppant, excitation of molecules of the shell, and consequent heatingof the shell by electromagnetic radiation, or inductive coupling of theshell to an external excitation field of alternating current. Thetemplate sphere can be formed by various processes which preferably makea template sphere having one or more voids. The template sphere can beformed by a spray drying process, a dehydrating gel process, a sol-gelprocess, a sol-gel-vibrational dropping process, a drop tower process, afluidized bed process, a coaxial nozzle gas-bubble process, athermolysis process, a chemical etching process, or a blowing process.Examples of these various processes are set forth below, and patents andapplications providing details of these processes which can be adaptedto the present invention are further provided below. It is noted that asan option the step of solidifying, hardening, or sintering to form thefinal template sphere can be optional, and the template sphere can beleft in a green state so that upon the formation or densification of theshell by sintering, the green template sphere can be also sintered,hardened, or otherwise solidified to form the proppant of the presentinvention.

The hollow template spheres can be made from aqueous or non-aqueoussuspensions or dispersions of finely divided inorganic or organic solidparticles, such as ceramic, glass, metal and metal glass particles,having particle diameters in the range of from about 0.01 to 10 microns(μm), a binder material, a film stabilizing agent, a dispersing agentfor the solid particles, and a continuous aqueous or non-aqueous liquidphase. The suspension or dispersion is blown into spheres using acoaxial blowing nozzle, and the spheres are heated to evaporate thesolvent and further heated or cooled to harden the spheres. The hardenedspheres are then subjected to elevated temperatures to decompose andremove the binder and any residual solvent or low boiling or meltingmaterials. The resulting porous hollow spheres are then fired at furtherelevated temperatures to cause the particles to sinter and/or fuse atthe points of contact of the particles with each other such that theparticles coalesce to form a strong rigid network (lattice structure) ofthe sintered-together particles.

A coaxial blowing nozzle and a blowing gas to blow hollow spheres from acontinuous liquid phase and dispersed particle film forming compositioncan be used and can comprise feeding the blowing gas to an inner coaxialnozzle, feeding the dispersed particle film forming composition to anouter coaxial nozzle, forming spherically shaped hollow spheres in theregion of the orifice of the coaxial blowing nozzle and removing thehollow spheres from the region of the orifice of the coaxial blowingnozzle. A transverse jet entraining fluid can be used to assist in thesphere formation and the detaching of the hollow spheres from theblowing nozzle. The continuous liquid phase of the dispersed particlefilm forming composition allows the hollow spheres to be blown byforming a stable film to contain the blowing gas while the hollow sphereis being blown and formed. The dispersed particles in the dispersedparticle composition, as the dispersed particle composition is formingthe hollow sphere and after the sphere is formed, link up with eachother to form a rigid or relatively rigid lattice work of dispersedparticles which dispersed particle lattice work with the binder andcontinuous liquid phase comprise the hollow green spheres. The hollowspheres, after they are formed, can be hardened in ambient atmosphere orby heating and removing a portion of the continuous phase. The hardenedhollow green spheres have sufficient strength for handling and furthertreatment without significant breaking or deforming of the microspheres.

The hardened green spheres can be treated at elevated temperatures toremove the remainder of the continuous liquid phase and volatilematerials such as binder, film stabilizing agent and dispersing agent.The treatment at elevated temperatures sinters and coalesces thedispersed solid particles to form rigid hollow porous spheres that canbe substantially spherical in shape, can have substantially uniformdiameters and can have substantially uniform wall thickness. The heatingat elevated temperatures, in removing the continuous phase and addedmaterials, creates interconnecting voids in the walls of the sphereswhich result in the porous characteristics of the spheres. The sinteringand coalescing of the dispersed solid particles, depending on the timeand temperature of the heating step, can cause a small degree ofcompaction of the dispersed particles and can cause the coalescing ofthe particles at the points in which they are in contact to form rigid,uniform size and shaped spheres of uniform wall thickness, uniform voidcontent and uniform distribution of voids in the walls and highstrength. Because the porosity is a result of the removal of thecontinuous phase from uniformly dispersed solid particles, the pores canbe continuous from the outer wall surface of the template sphere to theinner wall surface of the template sphere and the walls of the templatespheres can have substantially uniform void content and uniformdistribution of the voids that are created.

The hollow template spheres in general can be substantially spherical,have substantially uniform diameters, and have substantially uniformwall thickness and the walls have uniform void content and voiddistribution and voids which are connected to each other and to theinner and outer sphere wall surfaces. The walls of the hollow porousspheres can be free of latent solid or liquid blowing gas materials, andcan be substantially free of relatively thinned wall portions orsections and bubbles.

The hollow spheres can be made from a wide variety of film formingdispersed particle compositions, particularly dispersed ceramic, glass,metal, metal glass and plastic particle compositions and mixturesthereof. The dispersed particle compositions can comprise an aqueous ornonaqueous continuos liquid phase and have the necessary viscositieswhen being blown to form stable films. The hollow sphere stable filmwall after the sphere is formed rapidly changes from liquid to solid toform hollow green spheres. The hollow green spheres can be substantiallyspherical in shape and can be substantially uniform in diameter and wallthickness.

The hollow green spheres as they are being formed and/or after they areformed can have a portion of the continuous liquid phase removed fromthe dispersed particle composition from which the spheres were formed.The removal of continuous liquid phase can act to bring the dispersedparticles closer together and into point to point contact with eachother. The dispersed particles can then link up with each other to forma rigid or relatively rigid lattice work of dispersed particles whichparticles lattice work with the binder (if one is used) and continuousliquid phase (that remains) comprise the hollow green spheres. Thehollow green spheres are free of any latent solid or liquid blowing gasmaterials or latent blowing gases. The walls of the hollow green spheresare free or substantially free of any holes, relatively thinned wallportions or sections, trapped gas bubbles, or sufficient amounts ofdissolved gases to form bubbles. The term “latent” as applied to latentsolid or liquid blowing gas materials or latent blowing gases is arecognized term of art. The term “latent” in this context refers toblowing agents that are present in or added to glass, metal and plasticparticles. The glass, metal and plastic particles containing the “latentblowing agent” can be subsequently heated to vaporize and/or expand thelatent blowing agent to blow or “puff” the glass, metal or plasticparticles to form spheres. The hollow green spheres can have walls thatare substantially free of any holes, thinned sections, trapped gasbubbles, and/or sufficient amounts of dissolved gases to form trappedbubbles.

In general, the hollow template spheres can contain a single centralcavity, i.e. the single cavity is free of multiple wall or cellularstructures. The walls of the hollow spheres can be free of bubbles, e.g.foam sections. The hollow template spheres can be made in variousdiameters and wall thickness. The spheres can have an outer diameter of200 to 10,000 microns, preferably 500 to 6000 microns and morepreferably 1000 to 4000 microns. The spheres can have a wall thicknessof 1.0 to 1000 microns, preferably 5.0 to 400 microns and morepreferably 10 to 100 microns. When the dispersed particles are sintered,the smaller particles can be dissolved into the larger particles. Thesintered particles in the hollow porous spheres can be generally regularin shape and have a size of 0.1 to 60 microns, preferably 0.5 to 20microns, and more preferably 1 to 10 microns.

The ratio of the diameter to the wall thickness, and the conditions offiring and sintering the hollow template spheres can be selected suchthat the spheres are flexible, i.e., can be deformed a slight degreeunder pressure without breaking. The preferred embodiment of theinvention, particularly with the ceramic materials, is to select theratio of the diameter to wall thickness and the conditions of firing andsintering the hollow porous spheres such that rigid hollow porousspheres are obtained.

Another process to make the template sphere can involve a dehydratinggel process. An example of such a process can include the steps ofadding precursor material comprising an aqueous solution, dispersion orsol of one or more metal oxides (or compounds calcinable to metal oxide)to a liquid body of a dehydrating agent comprising an organicdehydrating liquid, agitating the liquid body to maintain the resultingdroplets of the precursor material in suspension and prevent settlingthereof, to maintain relatively anhydrous dehydrating liquid in contactwith the surface of the droplets as they are dehydrated, and to rapidlyextract within 30 seconds at ambient temperatures of 20° to 40° C., themajor amount of water from said droplets and form gelled microparticlestherefrom, the predominant amount of said gelled microparticles being inthe form of spherical, gelled, porous, liquid-filled spheres, recoveringsaid liquid-filled spheres, drying the resulting recovered spheres attemperatures and pressures adjusted to minimize fracture and burstingthe same and remove liquid from within the recovered spheres, and firingthe resulting dried spheres to form spherical ceramic spheres theperipheral wall or shell of each which encloses the single hollow withinthe interior thereof being porous and heat-sealable, homogeneous, andmade of non-vitreous ceramic comprising polycrystalline metal oxide oramorphous metal oxide convertible to polycrystalline metal oxide uponfiring at higher temperature.

Another method to make the template sphere can be a thermolysis process,such as the process described in U.S. Pat. No. 4,111,713, the disclosureof which is incorporated herein by reference. The method involves thepreparation of hollow spheres, the exterior wall of which comprises athermally fugitive binder material and sinterable inorganic particlesdispersed in the binder material. By “thermally fugitive” is meantmaterials that upon heating of the spheres will be removed from thespheres, e.g., by vaporization and/or oxidation or burning. Forinstance, at least 20% by volume of the thermally fugitive material canbe removed, or from 20% to 100% or 70% to 99% by volume. Natural orsynthetic organic materials which are readily burned such as corn starchsyrup, phenolic resins, acrylics and the like can be used as bindermaterials.

Besides a binder material, the solidifiable liquid sphere includes avolatile void-forming agent such as taught in U.S. Pat. No. 4,111,713.Other ingredients may also be included, such as a solvent or otherdispersing liquid. In addition, a metal or other inorganic material maybe included. Metallic binder combinations can be obtained by using (1) acolloidal dispersion of a metal, metalloid, metal oxide, or metal saltor (2) a metal, metalloid, metal oxide, or metal salt dispersion in aphenolic resin or other organic binder.

Typically, the solidifiable liquid sphere is formed at room temperature,e.g., by dissolving the binder material in a solvent or dispersing it inanother liquid. However, solid granules of binder material that becomeliquid during the tumbling operation may also be used.

During the sphere-forming operation the binder material should achieve aviscosity that is low enough for the parting agent particles to bewetted by the spheres, and preferably low enough so that any cellsforming inside an evacuated sphere will tend to at least partiallycoalesce, whereby binder material will be concentrated at the exteriorspherical wall or shell of the sphere. The parting agents can be appliedby a mixer or by a fluidized bed, using a similar approach that isdescribed in forming the shell. At the same time the viscosity of thebinder material should be high enough so that the expanded sphere willnot deform excessively while sphere formation is taking place. Theuseful range of viscosities for the binder material is broad, rangingfrom at least about 50 to 100,000 centipoises, but an especiallypreferred range is between about 100 and 10,000 centipoises. The spheresof binder material in the tumbling, sphere-forming operation are termedliquid herein, since even when at high viscosity they are flowable. Therange of useful viscosities will vary with particle size and the easewith which the parting agent particles can be wet. Surfactants can beused to advantage either as an ingredient in the binder material or as atreatment on the parting agent particle.

The parting agent particles used in practicing the invention can besolid discrete free-flowing particulate material which is sufficientlyinert, including sufficiently nonmelting, during the sphere-formingoperation to retain a parting function. In addition, parting agents thateventually become the primary or only constitutent of the sphere wallsshould be sinterable inorganic materials. Suitable metal parting agentsare iron, copper, nickel and the like. Suitable metalloid parting agentsinclude carbides such as silicon carbide, nitrides such as boronnitride, borides, silicides and sulfides. Suitable metal oxide partingagent particles include alumina, zirconia, magnetite, silica, mullite,magnesite, spinels and the like. Suitable metal salt parting agentparticles include metal hydroxides, nitrates, and carbonates.

Mixtures of different parting agent particles are used in someembodiments of the invention. For example, parting agent particlesproviding better flow properties, e.g., spheres, which may or may not besinterable, may be mixed with irregular sinterable parting agentparticles. Alternatively, mixtures are used to provide pigmentation,flame-retardancy, or variety in physical properties of the final sphere.However, sinterable particles generally constitute at least a majority,and preferably at least 60 volume percent, of the exterior wall of asphere so as to obtain adequate coherency and strength.

Generally, the parting agent particles can range from a few micrometersup to several hundred micrometers in size. They generally have adiameter no larger than the thickness of the wall of the final hollowsphere.

Generally, the solidifiable liquid spheres are used in sizes thatproduce hollow spheres about ½ millimeter to 2 centimeters in diameter.Spheres of the invention can be made with good uniformity of sizes byusing binder material granules or spheres of uniform size. Further, ofcourse, hollow spheres may be screened after formation to providedesired ranges of size.

The template spheres are generally round but need not be perfectlyspherical; they may be cratered or ellipsoidal, for example. Suchirregular, though generally round or spherical, hollow products areregarded as “spheres” herein.

The hollow template spheres can have a single hollow interior space,such as described in U.S. Pat. No. 4,111,713. The interior space in thesphere may be divided into a number of cells by interior walls havingessentially the same composition as the exterior wall; but even suchspheres have an outer wall, usually of rather constant thickness and ofgreater density, around the interior space. The outer wall is continuousand seamless (that is, without the junction lines resulting when twoseparately molded hemispheres are bonded together), though the wall maybe permeable or porous. The thickness of the outer wall is generallyless than about ½ the radius of the sphere and may be quite thin, asthin as 1/50 the radius, for example.

Another method to form the template sphere is also a dehydrating gel orliquid method which uses an aqueous precursor material that contains anaqueous solution, dispersion or sol of one or more metal oxides or metalcompounds calcinable to metal oxides, or mixtures of said forms ofprecursor materials. The precursor material should be pourable andstable, that is, non-gelled, non-flocculated or non-precipitated. Theequivalent concentration of the metal oxide in the precursor materialcan vary widely, e.g. a few tenths of one weight % to 40 or 50 weight %,and the particular concentration chosen will be dependent on theparticular form of the precursor metal oxide and dehydrating liquid usedand the desired dimensions and proposed utility of the template spheres.Generally, this concentration will be that sufficient to promote rapidformation of droplets in the dehydrating liquid and, generally, thelower the equivalent concentration of metal oxide in the precursormaterials, the thinner the walls and the smaller the diameters of thespheres.

The dehydrating liquid used to dehydratively gel the precursor materialis preferably a liquid in which water has a limited solubility and inwhich water is miscible to a limited extent. Such a dehydrating liquidwill practically instaneously cause formation of liquid droplets of theprecursor material and rapidly extract the major amount of the waterfrom the droplets to form discrete, dispersed, liquid-filled sphereshaving a porous gelled wall or shell, the physical integrity of which ismaintained in the body of dehydrating liquid. The formation of asubstantially quantitative yield of gelled spheres can be completewithin 30 seconds. Further, this formation does not require heating(i.e., it can be accomplished at ambient room temperature, e.g., 23° C.)nor does it require use of a barrier liquid. Though a small amount ofsolid beads may also be formed, the predominant amount, i.e., at least85-95% or higher, of the microparticles formed will be in the form oftemplate spheres. If the liquid-liquid extraction is carried out in abatch operation, there may be a tendency to form the small amount ofsolid beads (or relatively thicker-walled microcapsules) toward the endof the extraction due to the progressively decreasing dehydratingability of the dehydrating liquid as it extracts the water from theprecursor material.

Generally, dehydrating liquids can have a limited solubility of about 3to 50 weight %, preferably 15 to 40 weight % for water (based on theweight of the dehydrating liquid) at 23° C. Representative organicdehydrating liquids useful are alcohols, such as alkanols with 3-6carbon atoms, e.g. n-butanol, sec-butanol, 1-pentanol, 2-pentanol,3-methyl-2-butanol, 2-methyl-2-butanol, 3-methyl-3-pentanol,2-methyl-1-propanol, 2,3-dimethyl-2-butanol and 2-methyl-2-pentanol,cyclohexanol, ketones such as methyl ethyl ketone, amines such asdipropylamine, and esters such as methylacetate, and mixtures thereof.Some of these dehydrating liquids, e.g. n-butanol, when used to formspheres with relatively large diameters, e.g. 100-500 microns or larger,may have a tendency to cause micro-cracks in the walls of the spheres.Such micro-cracks can be prevented or minimized when such dehydratingliquids are used to form large spheres by adding a small amount of waterto such dehydrating liquids, e.g. 5 to 10% by weight of the dehydratingliquid. However, the resulting water-dehydrating liquid mixture stillhas the limited solubility for water, preferably at least 15 weight %.

The liquid-liquid extraction step can be carried out at ambienttemperatures, e.g. 20° to 40° C.; higher temperatures, e.g. 60° C. andhigher, cause fragmentation of the gelled spheres. Excellent,substantial, quantitative yields, e.g. 95% and higher, of gelledspheres, based on the equivalent oxide solids content of the precursormaterial, can be conveniently achieved at room temperature (23° C.). Inorder to quickly and efficiently dehydratively gel the droplets of theprecursor material in a batch operation, the body of dehydrating liquidcan be subjected to externally applied agitation (e.g. by swirling thebody of dehydrating liquid or by inserting a stirrer therein) when theprecursor material is added thereto, and the agitation is continuedduring the course of dehydration of the resultant droplets of precursormaterial. This agitation maintains the droplets in suspension (andthereby prevents agglomeration and settling of the droplets) and ensuresmaintenance of relatively anhydrous dehydrating liquid in contact withthe surface of the droplets as they are dehydrated. In a continuousliquid-liquid extraction operation, equivalent agitation can beaccomplished by adding the precursor material at a point to a stream ofthe dehydrating liquid flowing at a sufficient rate to maintain thedroplets in suspension in the course of their dehydration.

The dehydration of the droplets to form the gelled spheres can besufficiently complete within 30 seconds, and usually in less than 15seconds, from the time of addition of the precursor material, thataddition being in the form of drops, flowing stream, or by bulk.

The size of the droplets, and consequently the size of the resultantgelled and fired spheres, will be affected by the degree or type ofagitation of the dehydrating liquid as the precursor material is addedthereto. For example, with high shear agitation, e.g. that obtained witha Waring Blendor, relatively tiny droplets (and gelled spheres) can beformed, e.g. with diameters less than 20 microns. In general, gelledspheres with diameters in the range of about 1 to 1000 microns can beproduced.

The gelled, porous, transparent, liquid-filled spheres can be separatedand recovered from the dehydrating liquid in any suitable manner, e.g.by filtration, screening, decanting, and centrifuging, such separationbeing preferably performed soon after completion of the extraction step.Where the gelled spheres are recovered by filtration, filter cakecomprising the spheres and residual dehydrating liquid is obtained. Inany event, the recovered mass of gelled spheres are then sufficientlydried to remove the residual dehydrating liquid and the liquid withinthe spheres, the resultant dried, gelled spheres being convenientlyreferred to herein as green spheres, i.e. dried and unfired. The dryingcan be accomplished in any suitable manner, care being exercised toprevent too rapid an evaporation in order to minimize fracturing orbursting of the spheres. This drying can be carried out in ambient airand pressure in a partially enclosed vessel at temperatures, forexample, of 20°-25° C. Higher or lower drying temperatures can be usedwith commensurate adjustment of pressure if necessary to preventfracture of the wall of the spheres. During the course of drying, theliquid within the spheres diffuses through the shell or wall of thespheres, as evidenced by microscopic observation of the retreating uppersurface or meniscus of the liquid within the transparent spheres, thusattesting to the porous nature of the gelled spheres. The larger thedried spheres are, the more free-flowing they are. The dried sphereshave sufficient strength to permit subsequent handling. It may bedesired to screen classify them to obtain desired size fractions.

The dried spheres are then fired to convert them to spherical,smooth-surfaced, light weight or low density, rigid, crushable spheres,the shell or wall of which is non-vitreous, synthetic, ceramic,homogeneous, preferably transparent and clear, and comprises metal oxidewhich is polycrystalline or is amorphous metal oxide convertible topolycrystalline metal oxide upon firing at higher temperature. Dependingon the particular oxide precursor material and firing temperature used,the walls of the fired spheres will be porous and heat-sealable orimpermeable, the metal oxide in the walls being present in whole or inpart in the polycrystalline state or in an amorphous state capable ofconversion upon further firing to the polycrystalline state. Forexample, dried, gelled spheres made from Al₂O₃—B₂O₃—SiO₂ precursormaterial can be prefired at 500° C. to produce porous, transparent,ceramic spheres comprising amorphous Al₂O₃—B₂O₃—SiO₂, which can befurther fired at 1000° C. to form impermeable, transparent, ceramicspheres comprising polycrystalline aluminum borosilicate and anamorphous phase. As another example, dried, gelled spheres made fromTiO₂ precursor material can be prefired at 250°-450° C. to produceporous, transparent, ceramic spheres consisting of polycrystalline TiO₂,and these spheres can be further fired to or at 650° C. to formimpermeable, transparent, ceramic spheres consisting of anatase titania,TiO₂, and even further fired at 800° C. to form impermeable, ceramicspheres consisting of polycrystalline rutile TiO₂. The dried, gelledspheres can be fired in one step directly to impermeable spheres.

The template spheres can be prepared by a spray-drying process. Forinstance, spray-drying solutions can be used that contain nearly anyfilm-forming substance. Spray drying is particularly suited to thepreparation of hollow spheres from solids dispersed in aqueous media.U.S. Pat. Nos. 3,796,777; 3,794,503 and 3,888,957 disclose hollowspheres prepared by spray drying alkali metal silicate solutions thathave been combined with “polysalt” solutions, and then carefully dryingthe intermediate hollow spheres. The process by which these products aremade must be tightly controlled to minimize the holes, cracks and othersurface imperfections that contribute to porosity that is undesirable inthese products.

In general, largely spherical particles are produced from suchsubstances by forming a solution of the film-forming substance in avolatile solvent and spray drying that solution under conditions thatlead to the production of hollow particles of the size required. Asubstance that breaks down to provide a gas in the interior of theparticle may be required with certain systems to maintain the expansionof the product while it is still plastic and to prevent breakage underatmospheric pressure when the walls have set. Examples of useful blowingagents include inorganic and organic salts of carbonates, nitrites,carbamates, oxalates, formates, benzoates, sulfites and bicarbonatessuch as sodium bicarbonate, ammonium carbonate, magnesium oxalate, etc.Other organic substances are also useful, such as p-hydroxy phenylazide,di-N-nitropiperazines, polymethylene nitrosamines and many others.Selection of a particular blowing agent would be based uponcompatibility with the film-forming system and the intended use of theproduct.

Film-forming systems that are of particular value and which do notrequire the addition of a gas-forming substance as a blowing agent aredisclosed in U.S. Pat. No. 3,796,777, hereby incorporated by reference.Hollow spheres can be produced by forming a homogeneous aqueous solutioncomprising a sodium silicate and a polysalt selected from a groupconsisting of ammonium pentaborate, sodium pentaborate and sodiumhexametaphosphate (other inorganic materials can be used) and then spraydrying the solution under conditions necessary to produce hollow spheresof the size required. The spheres are further dried to reduce the watercontent and to set the walls. In general, the template spheres can havea bulk density of about 0.6 to 20 lbs/ft.³, a true particle density ofabout 2 to 40 lbs/ft.³ and a particle size of about 1 to 500 microns.

The film-forming system in which the organic solvent is used willdetermine the characteristics required, but in general it must be watermiscible and have a boiling point of 100° C. or more. Those solventsused with alkaline systems, such as those containing alkali metalsilicate, must be alkali stable and should not adversely affect thestability of the silicate solution. These characteristics need only befleeting, less than about 3 minutes, as the organic solvent need only beadded immediately before spray drying. In general, those organicsolvents that have a number of hydroxyl groups or exposed oxygens areuseful in the preferred alkali metal silicate polysalt combination.Examples of useful solvents include, among others, cellosolve,cellosolve acetate, ethyl cellosolve, diglyme and tetraglyme. About 0.5to 5.0 parts by weight of the solvent for each 100 pbw of the solids inthe feed solution are required to provide the beneficial effects of theimproved process.

The solution used to form hollow spheres in this method can contain 5 to50% of the film-forming solids. The amount of organic solvent additiveto achieve improved results is between 0.5 and 5%, so that between 0.025and 2.5% of the solution spray dried to form the hollow spheres issolvent. Ammonium pentaborate (APB), sodium pentaborate (SPB) and sodiumhexametaphosphate (SHP) can be used as “polysalts.” If a solution of APBand sodium silicate is used, the total solids would be 5 to 35% with 3to 15% as APB; the ratio of APB solids to sodium silicate solids shouldbe between 0.03:1.0 and 0.5:1.0 and preferably between 0.06:1.0 and0.5:1.0. About 0.015 to 1.75% of the organic solvents would be added tosuch solutions. A system having 0.02 to 0.3 parts by weight (pbw) of SPBper pbw of sodium silicate solids contains 17.4 to 34.5% total solidsand 6 to 7% SPB solids. This combination would require 0.087 to 1.7% ofthe appropriate organic solvent. A system having 1 to 3 pbw of SHP per 1pbw of silicate solids contains 29.6 to 48% of total solids. Thiscombination requires 0.14 to 2.4% of the organic solvent.

The process can be initiated by preparing a solution of the film-formingsolids in water, observing any required restrictions as toconcentration, order of addition, temperature or the like. It isimportant that any restrictions relating to viscosity are observed; ifthe viscosity of the solution is too high when spray dried, fibers mayresult. After the homogeneous solution is prepared, the organic solventis added with rapid agitation to ensure proper dispersion. The resultingmaterial is spray dried prior to any manifestation of instability suchas rising viscosity or gelling, such as spray dry within 10 minutes.

Any conventional spray drying equipment can be used to implement theprocess of this invention. The suspension-solution can be atomized intothe spray tower by either an atomizer wheel or a spray nozzle. Since awide range of film-forming materials and solvents can be used in thisprocess a wide range of spray drying temperatures can be used. Inlettemperatures of 50° to 500° C. can be used with outlet temperatures ofabout 40° to 300° C. Satisfactory products can be prepared from thefilm-forming system of sodium silicate and polysalt by spray drying thematerial at an inlet temperature of 200° to 500° C. and an outlettemperature of 100° to 300° C.

Another method of making the template sphere is by a blowing process orblowing agent process. An example of such a process is forming anaqueous mixture of inorganic primary component and a blowing agent. Themixture is dried and optionally ground to form an expandable precursor.Such a precursor is then fired with activation of the blowing agentbeing controlled such that it is activated within a predeterminedoptimal temperature range. Control of the blowing agent can beaccomplished via a variety of means including appropriate distributionthroughout the precursor, addition of a control agent into theprecursor, or modification of the firing conditions such as oxygendeficient or fuel rich environment, plasma heating, and the like.

The precursor for producing the expanded sphere can be produced bycombining the primary component, blowing component and optionally,control agent in an aqueous mixture. This aqueous mixture is then driedto produce an agglomerated precursor. As described above, a method offorming a precursor includes the steps of mixing and drying. Theresultant precursor is generally a substantially solid agglomeratemixture of its constituent materials.

The mixing step provides an aqueous dispersion or paste, which is laterdried. Mixing can be performed by any conventional means used to blendceramic powders. Examples of preferred mixing techniques include, butare not limited to, agitated tanks, ball mills, single and twin screwmixers, and attrition mills. Certain mixing aids such as, surfactantsmay be added in the mixing step, as appropriate. Surfactants, forexample, may be used to assist with mixing, suspending and dispersingthe particles.

Drying is typically performed at a temperature in the range of about 30to 600° C. and may occur over a period of up to about 48 hours,depending on the drying technique employed. Any type of dryercustomarily used in industry to dry slurries and pastes may be used.Drying may be performed in a batch process using, for example, astationary dish or container. Alternatively, drying may be performed ina spray dryer, fluid bed dryer, rotary dryer, rotating tray dryer orflash dryer.

The mixture can be dried such that the water content of the resultantagglomerate precursor is less than about 14 wt. %, more preferably lessthan about 10 wt. %, more preferably less than about 5 wt. %, and morepreferably about 3 wt. % or less. It was found that, in certainembodiments, with about 14 wt. % water or more in the precursor, theprecursor tends to burst into fines upon firing. This bursting can becaused by rapid steam explosion in the presence of too much water.Hence, in certain embodiments, the resultant precursor should preferablybe substantially dry, although a small amount of residual moisture maybe present after the solution-based process for its formation. In someembodiments, a small amount of water may help to bind particles in theprecursor together, especially in cases where particles in the precursorare water-reactive.

The dried precursor particles can have an average particle size in therange of about 10 to 1000 microns, more preferably about 30 to 1000microns, more preferably about 40 to 500 microns, and more preferablyabout 50 to 300 microns. The particle size of the precursor will berelated to the particle size of the resultant synthetic hollow templatesphere, although the degree of correspondence will, of course, only beapproximate. If necessary, standard comminuting/sizing/classificationtechniques may be employed to achieve the preferred average particlesize.

Drying can be performed using a spray dryer having an aqueous feed.Various techniques for controlling activation of the blowing agent canbe used such that it is activated at a pre-determined (e.g. optimaltemperature) point in the production process. Such control can beachieved by combining a control agent in the precursor formulation.Another embodiment includes a series of control agents and/or blowingagents such that there is sufficient blowing/expanding gas available atthe optimal temperature. In one embodiment, a series of blowing agentsmay be used which are sequentially activated as temperature rises.

Yet a further embodiment involves distributing the blowing agentthroughout the precursor such that while the precursor is being fired,the blowing agent distributed near the surface is exposed to a hightemperature but the blowing agent near the core of the precursor is“physically” protected. The thermal conductivity of the formulationcauses a delay between application of heat on the surface of theprecursor to temperature rise within the core of the precursor.Accordingly, blowing agent which is within the core of the precursorwill not be activated until a major portion of the precursor particlehas already reached its optimal temperature. Many blowing agents areactivated by oxidation. Particles within the core of the precursor willnot be exposed to oxygen to the same extent as blowing agent on thesurface, further protecting the blowing agent in the core of theparticle.

Spray dryers are described in a number of standard textbooks (e.g.Industrial Drying Equipment, C. M. van't Land; Handbook of IndustrialDrying 2^(nd) Edition, Arun S. Mujumbar) and will be well known to theskilled person.

The particle size and particle size distribution can be affected by oneor more of the following parameters in the spray drying process:

inlet slurry pressure and velocity (particle size tends to decrease withincreasing pressure);

design of the atomizer (rotary atomizer, pressure nozzle, two fluidnozzle or the like);

design of the gas inlet nozzle;

volume flow rate and flow pattern of gas; and

slurry viscosity and effective slurry surface tension.

The aqueous slurry feeding the spray dryer can comprise about 25 to 75%w/v solids, such as about 40 to 60% w/v solids.

In addition to the ingredients described above, the aqueous slurry maycontain further processing aids or additives to improve mixing,flowability or droplet formation in the spray dryer. Suitable additivesare well known in the spray drying art. Examples of such additives aresulphonates, glycol ethers, cellulose ethers and the like. These may becontained in the aqueous slurry in an amount ranging from about 0 to 5%w/v.

In the spray drying process, the aqueous slurry is typically pumped toan atomizer at a predetermined pressure and temperature to form slurrydroplets. The atomizer may be one or a combination of the following: anatomizer based on a rotary atomizer (centrifugal atomization), apressure nozzle (hydraulic atomization), or a two-fluid pressure nozzlewherein the slurry is mixed with another fluid (pneumatic atomization).

In order to ensure that the droplets formed are of a proper size, theatomizer may also be subjected to cyclic mechanical or sonic pulses. Theatomization may be performed from the top or from the bottom of thedryer chamber. The hot drying gas may be injected into the dryerco-current or counter-current to the direction of the spraying.

For example, a rotary atomizer has been found to produce a more uniformagglomerate particle size distribution than a pressure nozzle.Furthermore, rotating atomizers allow higher feed rates, suitable forabrasive materials, with negligible blockage or clogging. In someembodiments, a hybrid of known atomizing techniques may be used in orderto achieve agglomerate precursors having the desired characteristics.

The atomized droplets of slurry are dried in the spray dryer for apredetermined residence time. The residence time can affect the averageparticle size, the particle size distribution and the moisture contentof the resultant precursors. The residence time can be controlled togive the various characteristics of the precursor. The residence timecan be controlled by the water content of the slurry, the slurry dropletsize (total surface area), the drying gas inlet temperature and gas flowpattern within the spray dryer, and the particle flow path within thespray dryer. Preferably, the residence time in the spray dryer is in therange of about 0.1 to 10 seconds, although relatively long residencetimes of greater than about 2 seconds are generally more preferred.Preferably, the inlet temperature in the spray dryer is in the range ofabout 300 to 600° C. and the outlet temperature is in the range of about90 to 220° C.

Spray drying advantageously produces precursors having this narrowparticle size distribution. Consequently, synthetic expanded spheresresulting from these precursors can have a similarly narrow particlesize distribution and consistent properties for subsequent use.

The template sphere can be formed by a drop tower process, for instance,forming spheroidal particles from slag and ash. The spheroidal particlesare formed by dropping particles of slag and ash (or other inorganicmaterial) through a heated zone which fuses at least an outer surface ofthe particles. Any type of furnace can be used, such as a drop towerfurnace, a rotary kiln, a fluidized bed, and the like.

The process can involve spherulizing particles of coal slag oragglomerated coal fly ash, resulting from coal combustion (or usinginorganic or volcanic material). The process can include the steps of:

(a) providing a drop tube having an upper portion, a central portion anda lower portion;

(b) delivering a feedstock of particles to the upper portion of the droptube in a manner such that the particles flow in a substantiallyvertical downward path through the feed tube as individualizedparticles;

(c) heating the particles to a sufficient temperature by providing heatto the outer surface of the central portion of the drop tube to cause atleast the outer surface of the particles to melt such that a majority,i.e., at least about 50 weight percent, of the particles becomespheroidal due to surface tension at the outer surface; and

(d) cooling the particles, preferably in the lower portion of the droptube, to prevent agglomeration. Other types of furnaces can be used inthis embodiment or other embodiments, such as a tunnel furnace, amicrowave furnace and the like.

The slag or ash feedstock, which can range in size from, for example,about 0.001 to 10 mm, preferably from about 0.1 to 1 mm, can bedelivered through a feed tube having a discharge port, having one ormore holes, each with a diameter from, for example, at least the maximumparticle diameter of the feedstock, and more preferably, at least one totwenty times the maximum particle diameter of the feedstock, at thelower end thereof.

The template spheres can also be formed by chemical etching, such as byforming a sphere around a bead that is then subsequently removed by heator dissolving. At least 20% by volume of the bead can be removed ordissolved, such as from 20% to 100% or 70% to 99% by volume. The stepsin such methods comprise coating a polystyrene (or other polymer ordissolvable material) bead with an alumoxane solution (or otherinorganic or ceramic material), drying the bead, and then heating thecoated bead to a temperature sufficient to calcine the alumoxane toporous amorphous alumina (or other inorganic material). The coated beadis then washed in a solvent to remove the bead from inside the coating.The remaining shell is then heated to a temperature sufficient to forman α-alumina sphere. Besides alumoxane, other inorganic materials orsolutions can be used.

An alumoxane (A-alumoxane) can be prepared according to the methoddescribed in Chem. Mater. 9 (1997) 2418 by R. L. Callender, C. J.Harlan, N. M. Shapiro, C. D. Jones, D. L. Callahan, M. R. Wiesner, R.Cook, and A. R. Barron, which is incorporated herein by reference.Aqueous solutions of alumoxane can be degassed before use. Dry-formpolystyrene beads, such as those available from Polysciences, Inc., canbe used. Beads of polymers other than polystyrene may be used, so longas the polymer is soluble in a solvent. Likewise, beads of othermaterials may be used, so long as they are soluble in a solvent thatwill not damage the alumoxane coating.

The aqueous solution of A-alumoxane may range from 1-10 weight percent.The aqueous solution of A-alumoxane more preferably ranges from 2-8weight percent, and most preferably is 8 weight percent. Beads 10 mayrange from 1-80 μm in diameter, and are preferably 1-5 μm in diameterand more preferably about 3 μm in diameter.

The solution can be pipetted onto beads and then can be placed in acoated ceramic firing crucible, and allowed to dry in air. The coatingprocess can be conducted in a ceramic firing crucible to minimize theamount of agitation of beads. Beads can be covered or coated one tothree or more times to achieve a uniform alumoxane coating.

The alumoxane-coated polystyrene beads can be fired to 220° C. for 40minutes to burn off organic substituents. The firing converts thealumoxane coating to a porous amorphous alumina coating. This allows asolvent such as toluene to dissolve polystyrene beads but not theamorphous alumina coating. Beads with amorphous alumina coating can bestirred in toluene for 1 hour and then vacuum filtered. Multiple washescan be conducted to remove all of the polystyrene resulting from thedissolution of polystyrene beads, because the polystyrene solution tendsto “gum up” the surface of α-alumina sphere, precluding removal ofadditional polystyrene. To separate free-standing α-alumina spheres fromany extra alumina resulting from the coating process, the fired (1000°C.) material can be placed in water, centrifuged and filtered. Thecalcination temperature of 220° C. can be used.

A sol-gel process can be used to form the template sphere, such as byforming a metal oxide solution and adding a metal basic carbonate withacid and surface active agent and thickening agent to prepare a sol ofmetal, and then dropping the sol into an alkaline gelation bath and thenrinsing, drying, and calcining. Another method to form a template sphereinvolves the use of a sol-gel-vibrational dropping process, whereinaqueous solutions or sols of metal oxides, such as Hf or Zr, arepre-neutralized with ammonia and then pumped gently through a vibratingnozzle system, where, upon exiting the fluid stream, breaks up intouniform droplets. The surface tension of the droplets molds them intoperfect spheres in which gelation is induced during a short period offree fall. Solidification can be induced by drying, by cooling, or in anammonia, gaseous, or liquid medium through chemical reaction.

The following patents/applications, all incorporated in their entiretyby reference herein, provide examples of the above processes, which canbe adapted to form the template sphere of the present invention: U.S.Pat. Nos. 4,743,545; 4,671,909; 5,225,123; 5,397,759; 5,212,143;4,777,154; 4,303,732; 4,303,731; 4,303,730; 4,303,432; 4,303,431;4,744,831; 4,111,713; 4,349,456; 3,796,777; 3,960,583; 4,420,442;4,421,562; 5,534,348; 3,365,315; 5,750,459; 5,183,493; and U.S. PatentApplication Publication Nos: 2004/0262801; 2003/0180537; 2004/0224155.

The crush strength of the proppant can be determined in a uniaxialloading configuration in a strength testing cell with a cavity diameterof 0.5 inches (12.7 mm). The volume of material admitted to the interiorof the strength testing cell is 1.0±0.1 mL. Loading of the strength testcell is carried out using a Lloyd Instruments Compression Tester (ModelLR30K Plus) at a strain rate of 0.0400 inches per minute (1.016 mm perminute). The compressive force (lbf) and deflection in inches arerecorded continuously. The load at “failure” of the system under test iscalculated at a deflection of 0.02 inches (0.508 mm) from the appliedpreload value of 3.00 lbf. The uniaxial crush strength of the system(proppant, template, etc) is quoted in pounds per square inch (PSI) andcalculated from the load applied at 0.02 inches deflection divided bythe cross-sectional area of the strength test cell in square inches.

The template material or sphere can be made by hybrid methods based on acombination of two or more of the above-described exemplary methods formaking the template material or sphere. For instance, a template spherehaving voids can be formed by way of a coaxial blowing process and thena drop tower design can be used to form the spheres. For instance, thedrop tower can convert the template material into a sphere or a morespheroidal shape through the drop tower approach, and/or the drop towerapproach can permit a cooling of the template material or sphere.

In the present invention, the present invention provides improvementswith respect to proppant technology. Currently, there is a balance ofproperties that must be met, such as with respect to specific gravity orbuoyancy and sufficient crush strength. In the past, if one wanted toachieve a proppant having sufficient crush strength, the specificgravity and density of the overall proppant was too high such that theproppant would be difficult to pump to the particular location in thesubterranean formation or, when in the subterranean formation, theproppant would not be uniformly distributed since the proppant was tooheavy and would sink in the medium used to transport the proppant. Onthe other hand, some proppants may have sufficient low specific gravity,meaning that the proppant would satisfy buoyancy requirements, however,by doing so, the proppant typically does not have reliable crushstrength and, therefore, the proppant would fail (e.g., deform, fractureor break) once in the subterranean formation, if not earlier. Thepresent invention achieves the desirable balance of properties by, in atleast some embodiments, using a template sphere or material which haslow density or desirable specific gravities and then strengthening thetemplate by providing a shell around the template sphere therebycreating sufficient crush strength to the overall proppant due to theshell. Thus, in the present invention, at least in one embodiment, thetemplate sphere provides the desirable buoyancy or specific gravityrequirements, and the shell of the present invention provides thedesirable crush strength and related properties. A balance of competingproperties is achievable by the present invention.

The template or substrate (e.g., template sphere) and/or one layer (ormore layers, if present) comprising the shell can contain fugitivephases and/or hollow material (e.g., hollow spheres, hollowparticulates, hollow materials having other shapes, wherein the hollowmaterial can have one central void and/or multiple voids or cells). Thefugitive phase or phases can be formed from material that can burn outor otherwise be removed at some point in the formation of the proppant,if desirable, such as during the sintering step described previously.For instance, the fugitive phases can include, but are not limited to,volatile material, vaporizable material, combustible material, and/orthe like. Specific examples include, but are not limited to,cellulose-based material, wood-based material, carbon-based material,such as carbon black, carbon fibers, charcoal, activated carbon, coal,paper, plant material, flour, other particulates that are combustiblesuch as some of the material mentioned previously that has thisproperty, and the like. The fugitive phases can be uniformly present inone layer (or more than one layer if present) that comprises the shelland in addition, or in the alternative, can be present in the templateor substrate, wherein the fugitive phases can be uniformly distributedthroughout the template or layer or can be located in specificlocations, such as a central area, edge, upper surface, or otherlocations. The fugitive phases may also be present in a non-uniformdistribution, in that there is a graded concentration of the fugitivephase through the bulk of the shell, with a higher concentration offugitive phase near the surface regions and a lower concentration nearthe internal regions (or surfaces). The opposite distribution may alsobe used as may an increase in concentration of the fugitive phase in thecenter regions of the shell and a lower concentration at the internaland outer surfaces. These fugitive phases can then form void areas uponthe fugitive material being optionally burned out or otherwise removedby other means (e.g., chemical dissolution). When multiple layers areused to form or comprise the shell, one of the layers can contain afugitive phase, whereas a second or third layer does not contain thefugitive phase, or the number of fugitive phases, the location of thefugitive phases, and/or the size of the fugitive phases can be the sameor different with respect to when the fugitive phase is present in twoor more layers or is present in one or more layers of the shell and thetemplate. Similarly, hollow material, such as hollow spheres, such ascenospheres or any of the hollow material mentioned above (e.g., ceramicmicrospheres, ceramic nanospheres, glass microspheres, glassnanospheres, expanded perlite, etc) can be present in the templateand/or one layer (or more than one layer if present) that can comprisethe shell. Like the fugitive phase, the hollow material can be the sameor different if present in more than one layer comprising the shell orif present in one or more layers of the shell and the template. Thenumber of hollow material, the location of the hollow material, and/orthe size of the hollow material and/or the amount of the hollow materialcan be the same or different per layer or for the one or more layers ofthe shell and template. As with the fugitive phases, the hollow materialcan be centrally located in the template or can be uniformly distributedthroughout the template or layer or can be specifically located incertain portions of the template or layer, such as the outer area, innerarea, and the like. Similarly, with respect to the one or more layers ofthe shell that contains hollow material, the hollow material can beuniformly distributed throughout the shell or it can be located incertain portions of the shell, such as the outer area or inner area ofthe one or more layers comprising the shell. The amount of fugitivephases and/or microspheres or other hollow material can be any volumepercentage based on the volume of the layer or template containing thismaterial. For instance, the fugitive phases or hollow material can bepresent in a volume amount of from about 0% to about 15% by volume(e.g., 0.1% to 10%, 0.5% to 7%, 1% to 5% by volume) and, at this amount,the overall proppant can be significantly lightened and/or toughenedfrom the standpoint of crush strength due to this amount. When amountsgreater than 15% by volume are present in a layer or template, such as15% by volume to 80% by volume (e.g., 20% to 70%, or 25% to 60% or 30%to 50% and the like), this amount typically leads to strictly alightening effect (e.g., overall lowering of density and/or weight ofthe proppant) as opposed to any strengthening effect. Thus, when amountsgreater than 15% by volume are present in one layer or the template orboth, a lightening effect or scaffolding regime (open-cell network,which can improve overall sphericity and surface smoothness, but notimpart mechanical strength) can be achieved. However, a layer(s) cancomprise from about 15% to about 25% by volume of the hollow materialwhen the diameter size of the hollow material is from 0.1 to 0.2microns. Further, as an option, the size diameter of the fugitive phasematerial can be from 0.2 to 2 microns such as 0.2 to 1 micron. Thehollow material can be smaller in size than fugitive phase material. Thesizes can be, for instance from 0.1 microns and 0.2 microns, and 10microns or greater. The hollow material can be obtained from: ApolloSRI, and Nanoridge Materials, or formed following U.S. Pat. No.7,220,454, incorporated herein by reference.

As an option, coatings, such as resin coatings, can be used as one ofthe layers present on top of the template or substrate or present on oneor more of the layers comprising the shell. This coating can have theability to fill in at least a part of the voids formed from the fugitivephases and/or can be used to reduce porosity. The coatings, such asresin coatings or other material that have the ability to fill voids,can be used after the fugitive phase is burned out (or otherwiseremoved) or prior to the fugitive phase being burned out (or otherwiseremoved). Most preferably, the resin coating is applied after burn-outor removal of the fugitive phase. In general, the fugitive phase can beentirely removed by burn-out or other means or it can be partiallyremoved by burn-out or other means, depending upon the particular usesof the proppant. Preferably, the removal of the fugitive phase can be100% or nearly 100%, such as from 90% to 99% by volume, though amountsless than 90% by volume can be achieved if desired. As an option, whenthe fugitive phase and/or hollow material is present in more than onelayer or in a layer(s) and the template, the amounts and type offugitive phase material and/or hollow material can be present indifferent volume amounts. For instance, the template or substrate cancontain a high level by volume of a fugitive phase and/or hollowmaterial, and one or multiple layers comprising the shell can containlower amounts of the hollow material and/or fugitive phase in order toensure that the layer has sufficient strength, such as crush strength.Thus, as a specific example, the template or substrate can contain 0% to80% by volume of a fugitive phase and/or hollow material, such as 15% to80% by volume (or 20 vol % to 70 vol %, or 30 vol % to 60 vol %),whereas one or more layers on the template that form the shell cancontain hollow material and/or fugitive phases in an amount of from 0%to 15% by volume (based on the volume of the layer that the fugitivephase and/or hollow material is present within). The fugitive phasesand/or hollow material can be present on the surface of the one or morelayers comprising the shell and/or the template surface and/or can bepresent within the thickness of the layer comprising the shell and/ortemplate sphere.

The use of fugitive phases and/or hollow materials can permit the use ofless strength-bearing phases (e.g., alumina, cordierite, mullite,steatite, zirconia, metal oxides, ceramics, etc), thus reducing thecosts, as well as improving control over other attributes, such asmaterial toughness, strength, strain fields, elastic moduli, compliancy,and the like, which can result in the optimization of the overallstrength-to-weight ratio.

As an example, when more than one layer contains a hollow materialand/or fugitive phase, the amount of hollow and/or fugitive phasematerial can decrease as one goes from the inner layers that comprisethe shell to the outer layers. In other words, the outer layers, in oneembodiment, can contain less fugitive phase material and/or hollowmaterial than the inner layers that comprise the shell and/or thetemplate or substrate itself. This can be achieved through acontinuously rated percentile of fugitive phase and/or hollow materialby volume. These various embodiments can be present in any of theembodiments described throughout the present application in combinationwith the other embodiments or as separate embodiments. The amount of thecoating, such as resin coating, can be present in an amount of fromabout 1 vol % to about 20 vol %. The vol % binder is based up the volumeof the proppant (e.g. sphere). In the ideal case the resin layer coatsthe surface of the proppant thus sealing any porosity off from themigration of fluids etc. There may be some migration of the resinthrough the porosity into the interior regions.

The fugitive phase can be “burned” out during the heating cycle andusually occurs at temperatures less than that of sintering. In the caseof carbon based fugitive phases the decomposition temperature can befrom about 400 deg C. to about 800 deg C. in an oxidizing atmosphere(the temperature being dependent upon the fugitive phase). Othertemperatures can be used. The temperatures herein are a reference to thetemperature of the material being subjected to the heating. The actualtime for “burn-out” of the fugitive phase may vary from 15 minutes up to60 minutes or more. Alternatively, the heating rate may be slowed fromabout 400 deg C. to about 800 deg C. to allow time for “burn-out” tooccur without the requirement for a dwell time at a specifictemperature. The use of hollow material like nano or micro spheres ispreferential to the use of a fugitive phase as the voids thus generatedare able to persist in the structure during sintering. The poresgenerated via the fugitive phase methodology may be eliminated duringsintering due to the effects of grain growth and grain migration. In thecase of hollow material like spheres, the pores will tend to persistduring sintering due to the lower specific surface energy of the poregeometry and the fact that the sphere material provides a barrier tograin growth and grain migration. The size of the fugitive phases and/orhollow material that can be used can vary in diameter size of from about0.1 μm to about 10 μm, such as from about 0.1 μm to about 0.5 μm. Othersizes can be used.

The template and/or one or more layers comprising the shell, can containany amount of Steatite, such as amount (vol %) of from 75 vol % to about100 vol %. The Steatite can be used in combination with other materialsdescribed previously in the present invention.

The reinforcing particulates, reinforcing material and/or any of thecomponents used to form one or more layers of the shell and/or templatecan be treated with one or more of the surface treatments mentioned,such as the heat treatment, densification, surface modifications,coatings, glazings, mechanical treatment, chemical treatment, a directparticle deposition from a slurry containing precursor particles,preferential dissolution of one or more species on the particulates orcomponents, one or more chemical treatments, one or more mechanicaltreatments, one or more thermal treatments, one or more spark dischargetreatments, or any combination thereof, and the like.

For purposes of the present invention, the template sphere (or templateor substrate) can be used alone, meaning no shell is present. Forpurposes of the present invention, the template sphere will be referredto, but it is to be understood that the embodiment(s) further relates tothe template material in general or the substrate as referred to herein.The template sphere can be used in the same applications as the templatewith shell described herein. For instance, the template sphere can beused alone, without a shell, as a proppant for propping open formations,in hydrocarbon recovery, as part of a component in concrete, as well asthe other applications described herein. The template sphere can be anyof the template spheres described herein. The template sphere can be atemplate sphere wherein one or more flaws on the surface of the templatesphere are removed or diminished as described herein. As mentionedalready, these flaws can be removed by one or more chemical treatments,one or more mechanical treatments, one or more thermal treatments, oneor more spark discharge treatments, or any combination thereof. Thevarious treatments that can be used as described above can be used toform the sphere alone and/or to modify the surface of the sphere forpurposes of using the template sphere alone or with a coating or shell.

The silica migration (and/or other metal oxide migration) can becontrolled, such as by two methods, i.e. reaction of silica (or othermetal oxide) and the formation of an impervious boundary phase topreclude further migration of the silica (or other metal oxide). In thepresent invention, a thin layer (<20 μm) of a calcium containing species(e.g. CaCO₃, CaO, etc) can be coated over the template, the shell (or atleast one layer of the shell if multiple layers are present), and/orreinforcing material with subsequent heat treatment. The reactionbetween the silica (or other metal oxide) and Ca containing compound canproduce a range of calcium silicate phases (CS, C₃S, CS₃, CS₆, etc) orother calcium phases, which can then hydrate in the presence of moistureto form a dense impervious layer. A subsequent over coating(s) with astrength bearing layer(s), such as a strength bearing ceramic coating,can yield a sintered microstructure that can be essentially devoid offree silica at the grain boundaries, thus yielding a significantimprovement in materials properties.

Theoretical Examples

A. A mixture of 70 vol % sub-micron aluminum nitride powder (meanparticle size of 0.3±0.1 μm) and a sub-micron aluminum metal powder(mean particle size of 0.3±0.1 μm) was ball milled for 12 hours inmethanol to achieve intimate mixing of the powders. An organic binder(10 wt % Poly vinyl alcohol in methanol) was added to thepowder—methanol mixture to improve the green strength of theceramic—metal mixture prior to heat treatment. The powder mixture wasdried and then pressed into compacts of approximately 12 mm diameter by6 mm high and cold isostatically pressed at 150 MPa for 5 minutes. Thegreen compacts were placed in a controlled atmosphere furnace and thechamber was evacuated and then purge with argon to reduce the oxygenconcentration to a minimum. The samples were then heated at a rate of 5°C.min⁻¹ to 650° C. and held at this temperature for 60 minutes to effectmelting of the aluminum metal phase and allow infiltration of the moltenaluminum through the pore structure of the ceramic perform. The sampleswere then heated at the same rate to 1200° C., and the furnaceatmosphere was purged with a blend of anhydrous ammonia gas and hydrogengas (98% anhydrous ammonia, 2% hydrogen) and the pressure raised toapproximately 300 kPa. The samples were held at this temperature andpressure for 12 hours to effect nitridation of the aluminum metal andthen cooled at approximately 10° C.min⁻¹ to ambient temperature.

B. Using the same composition as Example A, the slurry was spray coatedonto shape forming templates (cenospheres with a mean particle size of200±20 μm) in a fluid bed spray coater. The spray coater was operatedaccording to the following base parameters, Air temperature: 40°-90° C.,Airflow: 90-150 liters per minute, Nozzle Air Setting: 10-25 psi. Theslurry flow rate was varied to maintain the specified droplet size. Thecoating process was continued until the outer diameter of the spheresincreased to 350±25 μm, yielding a shell thickness of approximately75±10 μm. The coated templates were then heat treated in the same manneras presented in Example A.

C. Using the same composition as Example A, the powder slurry was driedand then admitted to the process chamber of a granulator. The granulatorwas operated until spherical bodies were produced with an approximateouter diameter of 350±40 μm. The solid spherical granulates were thenremoved from the process chamber and subjected to the same thermaltreatments as presented in Example A.

D. A mixture of 70 vol % sub-micron silicon carbide powder (meanparticle size of 0.3±0.1 μm) and a sub-micron silicon metal powder (meanparticle size of 0.3±0.1 μm) was ball milled for 12 hours in methanol toachieve intimate mixing of the powders. An organic binder (10 wt % Polyvinyl alcohol in methanol) was added to the powder—methanol mixture toimprove the green strength of the ceramic—metal mixture prior to heattreatment. The powder slurry was dried and then admitted to the processchamber of a granulator. In addition to the powder formulation,spherical seed templates of starch with an approximate mean diameter of200±20 μm were added to the process chamber. These spherical seedtemplates were coated with the powder formulation during the granulationprocess. The granulator was operated until spherical bodies wereproduced with an approximate outer diameter of 350±30 μm, yielding agreen ceramic—metal composite shell over the seed templates with anapproximate wall thickness of 75±15 μm. The solid spherical granulateswere then removed from the process chamber and subjected to a primarythermal treatment that involved heating the samples at 2° C.min⁻¹ to atemperature of 450° C. and held at this temperature for 240 minutes toeffect pyrolization of the starch seed particle. Following this, thefurnace chamber was purged with argon gas and the samples heated at arate of 5° C.min⁻¹ to a temperature of 1600° C. Upon reaching thistemperature, the furnace chamber atmosphere was purged with carbonmonoxide gas and pressurized to approximately 400 kPa. The samples wereheld at this temperature and pressure for 12 hours to convert thesilicon metal to silicon carbide. Following this, the furnace was cooledat approximately 10° C.min⁻¹ to ambient temperature.

E. Using the same composition as Example A, the prepared slurry isadmitted to the slurry supply tank of a coaxial nozzle system. Theslurry was allowed to travel down through the nozzle and blowing airpressure was admitted to the central conduit of the coaxial nozzlesystem. The pumping rate of the slurry was set such that in combinationwith the blowing air pressure a hollow sphere with an approximatediameter of 400±20 μM was formed at the tip of the nozzle. The slurryflow and blowing air was interrupted momentarily to allow the hollowdroplet to stabilize and then the nozzle was vibrated to allow thedroplet to detach and free fall through the setting solution, andcollected in a vessel at the lower portion of the column. The slurryflow and blowing air was restarted to form another hollow sphere. Theprocess is continued until the volume of slurry is consumed. The hollowspheres are dried to remove any residual organic materials and thensubjected to the same thermal treatments as Example A.

The present invention is not only limited to the fabrication ofproppants, but may also be applied to filler materials for cements,cement fiber board systems, drywall fillers, caulks, polymeric systemsand other such applications that require high strength, low densityfiller materials.

Applicants specifically incorporate the entire contents of all citedreferences in this disclosure. Further, when an amount, concentration,or other value or parameter is given as either a range, preferred range,or a list of upper preferable values and lower preferable values, thisis to be understood as specifically disclosing all ranges formed fromany pair of any upper range limit or preferred value and any lower rangelimit or preferred value, regardless of whether ranges are separatelydisclosed. Where a range of numerical values is recited herein, unlessotherwise stated, the range is intended to include the endpointsthereof, and all integers and fractions within the range. It is notintended that the scope of the invention be limited to the specificvalues recited when defining a range.

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the present specification andpractice of the present invention disclosed herein. It is intended thatthe present specification and examples be considered as exemplary onlywith a true scope and spirit of the invention being indicated by thefollowing claims and equivalents thereof.

1. A method of making a proppant from a proppant material comprising atleast one metal oxide, said method comprising reducing in a reactor atleast a portion of the at least one metal oxide to respective metal andnitriding and/or carbiding at least a portion of said respective metalto form said proppant.
 2. The method of claim 1, wherein said proppantmaterial is at least one metal oxide, glass spheres, or cenospheres. 3.The method of claim 1, wherein said proppant material is in the shape ofa solid sphere, hollow sphere, sphere having multiple voids, or anycombination thereof.
 4. The method of claim 1, wherein said proppantmaterial has an outer surface comprising at least one metal oxide andsaid nitriding occurs on at least said outer surface.
 5. The method ofclaim 4, wherein said nitriding forms a nitride layer or phase.
 6. Themethod of claim 4, wherein said carbiding forms a carbide layer orphase.
 7. The method of claim 4, wherein the entire outer surface isnitrided to form a nitride layer or phase.
 8. The method of claim 4,wherein the entire outer surface is carbided to form a carbide layer orphase.
 9. The method of claim 1, wherein all of the at least one metaloxide present in the proppant material is reduced to said respectivemetal.
 10. The method of claim 9, wherein said respective metal isnitrided or carbided.
 11. The method of claim 1, wherein said reducingoccurs in a fluidized bed furnace, static bed furnace, rotary furnace,microwave furnace, drop tower furnace, or muffle furnace.
 12. The methodof claim 1, wherein said reducing comprises utilizing at least onereducing agent in the presence of said proppant material.
 13. The methodof claim 12, wherein said reducing agent is carbon monoxide.
 14. Themethod of claim 1, wherein said nitriding comprises nitriding withnitrogen gas, anhydrous ammonia gas, nitrogen oxide, and optionally withhydrogen gas or helium gas or both.
 15. The method of claim 11, whereinsaid method further comprises utilizing air as a fluidizing medium. 16.The method of claim 15, wherein said air is pre-heated.
 17. The methodof claim 1, wherein said method further comprises introducing acarbonaceous material before, during, and/or after said reducing. 18.The method of claim 17, wherein said carbonaceous material ispulverized.
 19. The method of claim 17, wherein said carbonaceousmaterial is petroleum coke, charcoal, graphite, or any combinationthereof.
 20. The method of claim 1, wherein said reducing comprisesintroducing at least one reactant gas to said reactor.
 21. The method ofclaim 20, wherein said reactant gas at least partially replaces afluidizing medium that is present in the reactor.
 22. The method ofclaim 1, wherein said nitriding comprises introducing at least onenitrogen-containing gas into said reactor.
 23. The method of claim 22,wherein said nitrogen-containing gas replaces said reactant gas.
 24. Themethod of claim 17, wherein said carbonaceous material is at leastpartially combusted during said method.
 25. The method of claim 24,wherein prior to or during said reducing, an oxygen-containing gas isintroduced into said reactor to partially combust said carbonaceousmaterial.
 26. The method of claim 17, wherein at least a portion of saidcarbonaceous material reacts with at least a portion of said metal toform a metal-carbide phase.
 27. A proppant produced by the method ofclaim
 1. 28-88. (canceled)
 89. A method of making a proppant from aproppant material comprising at least one metal oxide or metal or both,said method comprising nitriding or carbiding at least a portion of saidat least one metal oxide or metal or both to form said proppant.
 90. Themethod of claim 89, wherein said carbiding occurs at a temperature ofless than about 1800° C.
 91. The method of claim 1, wherein said metaloxide is silica, and said metal is silicon.
 92. The method of claim 1,wherein said proppant material further comprises at least one metalphase, wherein at least a portion of said metal phase is directlynitrided and/or carbided.
 93. The method of claim 1, wherein saidproppant material comprises one or more of an oxide of aluminum,antimony, boron, cadmium, cobalt, gold, iron, lead, lithium, magnesium,molybdenum, nickel, platinum, potassium, silver, tin, titanium,tungsten, zinc, or zirconium or any combination thereof.
 94. The methodof claim 1, wherein said proppant material comprises a metal ofaluminum, antimony, boron, cadmium, cobalt, gold, iron, lead, lithium,magnesium, molybdenum, nickel, platinum, potassium, silver, tin,titanium, tungsten, zinc, or zirconium or any combination thereof. 95.The method of claim 1, wherein the metallic carbide phase is formeddirectly from the metallic oxide phase without having to form themetallic phase first.
 96. The method of claim 12, wherein a fluidizingmedium is present and comprises a non-oxidizing, oxygen-free fluid,which is optionally pre-heated.
 97. The method of claim 17, wherein thecarbonaceous material is added to the metal oxide at the commencement ofthe reaction.
 98. The method of claim 17, wherein the carbonaceousmaterial is added to the metal oxide at the commencement of the reactionprocess in excess of that required for stoichiometry.
 99. The method ofclaim 17, wherein the carbonaceous material is added to the reactionchamber with the metal oxide to cause the reduction of the metal oxideto the metal, and upon completion of the reduction reaction, furthercarbonaceous material is added to the reaction chamber to form the metalcarbide.
 100. The method of claim 17, wherein the carbonaceous materialis derived from one or more biomass materials.
 101. The method of claim17, wherein the carbonaceous material at least partially reacts withoxygen atoms present in the metal oxide. 102-103. (canceled)
 104. Themethod of claim 1, wherein the metal oxide is deposited on a template,wherein the template is pyrolyzed and forms a carbonaceous material thatreacts with the metal oxide to form a metal-carbide phase from internalregions of the proppant towards the outer surface.
 105. The method ofclaim 104, wherein the template is a polymer, petroleum coke, charcoal,graphite, or any combination thereof.
 106. The method of claim 104,wherein the template material is a biomass material. 107-113. (canceled)114. The method of claim 1, wherein said proppant has an outer surfaceand said method further comprises treating said proppant to improve thehydrophobic nature of the proppant.
 115. The method of claim 1, whereinsaid proppant has an outer surface and said method further comprisestreating said proppant to improve the hydrophilic nature of theproppant.
 116. The method of claim 1, wherein said proppant has an outersurface and said method further comprises treating said proppant toproduce a hydro neutral surface on the proppant. 117-121. (canceled)